Return a promise when calling PSPDFKit.present()

[irgendeinich]

Resolves #243

Details

Calling present() now returns a promise that either resolves to true or provides the exception that occurred when loading the document.

Acceptance Criteria

• When approved, right before merging, rebase with master and increment the package version in package.json, package-lock.json, samples/Catalog/package.json, and samples/NativeCatalog/package.json (see example commit: 1bf805f).
• Create a new release (and tag) with the new package version (see https://github.com/PSPDFKit/react-native/releases).

[radazzouz]

@steviki @irgendeinich I just pushed the iOS changes in 83cacd7

[steviki]

iOS part looks good to me!