Add support for backing up cards to CSV file on external storage

[brarcher]

This pull request adds support for importing/exporting the card information from/to CSV.
#36
#20

Additionally, an About page is added.
#19

[brarcher]

@airon90 and @PanderMusubi, this pull request adds several more strings which need to be translated. Would you mind providing the appropriate translations for Italian and Dutch when there is time? The strings are as follows:

    <string name="importExport">Import/Export</string>
    <string name="importName">Import</string>
    <string name="exportName">Export</string>
    <string name="importExportHelp">Data is imported to/exported from LoyaltyCardLocker.csv on external storage</string>
    <string name="importedFrom">Imported from: %1$s</string>
    <string name="exportedTo">Exported to: %1$s</string>
    <string name="fileMissing">File missing: %1$s</string>
    <string name="importFailed">Failed to import: %1$s</string>
    <string name="exportFailed">Failed to export: %1$s</string>
    <string name="importing">Importing&#8230;</string>
    <string name="exporting">Exporting&#8230;</string>
    <string name="noExternalStoragePermissionError">Unable to import or export cards without the external storage permission</string>
    <string name="about">About</string>
    <string name="app_copyright_fmt">Copyright 2016-<xliff:g>%s</xliff:g> Branden Archer</string>
    <string name="app_license">Licensed under the GPLv3.</string>
    <string name="about_title_fmt">About <xliff:g id="app_name">%s</xliff:g></string>
    <string name="debug_version_fmt">Version: <xliff:g id="version">%s</xliff:g></string>
    <string name="app_revision_fmt">Revision Information: <xliff:g id="app_revision_url">%s</xliff:g></string>
    <string name="app_libraries"><xliff:g id="app_name">%s</xliff:g> uses the following third-party libraries: <xliff:g id="app_libraries_list">%s</xliff:g></string>
    <string name="ok">OK</string>

Thanks!

[PanderMusubi]

<string name="importExport">Importeer/Exporteer</string>
<string name="importName">Importeer</string>
<string name="exportName">Exporteer</string>
<string name="importExportHelp">Data is geïmporteerd van of geëxporteerd naar LoyaltyCardLocker.csv op externe opslag</string>
<string name="importedFrom">Geïmporteerd van: %1$s</string>
<string name="exportedTo">Geëxporteerd naar: %1$s</string>
<string name="fileMissing">Bestand niet gevonden: %1$s</string>
<string name="importFailed">Importeren mislukte: %1$s</string>
<string name="exportFailed">Exporteren mislukte: %1$s</string>
<string [name="importing">Importerende&#8230;</string>
<string name="exporting">Exporterende&#8230;</string>
<string name="noExternalStoragePermissionError">Niet mogelijk te importeren of exporteren zonder rechten op externe opslag</string>
<string name="about">Over</string>
<string name="app_copyright_fmt">Auteursrecht 2016-<xliff:g>%s</xliff:g> Branden Archer</string>
<string name="app_license">Gelicenseerd met GPLv3.</string>
<string name="about_title_fmt">Over <xliff:g id="app_name">%s</xliff:g></string>
<string name="debug_version_fmt">Versie: <xliff:g id="version">%s</xliff:g></string>
<string name="app_revision_fmt">Revisieïnformatie: <xliff:g id="app_revision_url">%s</xliff:g></string>
<string name="app_libraries"><xliff:g id="app_name">%s</xliff:g> gebruikt de volgende bibliotheken van derden: <xliff:g id="app_libraries_list">%s</xliff:g></string>
<string name="ok">Oké</string>

[brarcher]

Thanks for the translations, @PanderMusubi and @airon90 (from #39)!