Added properties for base64 formatting

[kuzjka]

New feature

This PR continues the mailing list thread and suggests system properties to control base64Binary values formatting:

• org.apache.xml.security.base64.lineSeparator, org.apache.xml.security.base64.lineLength - allows to override default MIME encoding settings
• org.apache.xml.security.base64.ignoreLineBreaks - disables line wrapping for base64 value, but allows to keep the whole XML pretty-printed. Takes precedence over line wrapping settings above.
• existing org.apache.xml.security.ignoreLineBreaks takes precedence over all new options.

Default values result to CRLF line breaks with 76 chars in the line, making the whole thing fully compatible with previous implementation.

Other changes

Some of the logic is included to XMLUtils to provide better encapsulation:

• adding line breaks in the beginning and in the end of wrapped base64 values in the element is implemented as XMLUtils.encodeElementValue
• Base64OutputStream from Commons, used in XML encryption, is replaced with a java.util implementation and provided by XMLUtils, making it consistent with configured Base64 encoder.

Base64.Encoder/Decoder instances are thread-safe, so sharing a single instance should give us a little performance gain.

Test coverage

As the base64 configuration takes place during class load, unit-tests leverage from custom ClassLoader and Reflection API to reinitialize the class in each test.

Motivation

https://www.w3.org/TR/xmlschema-2/#base64Binary - base64Binary definition has note, that original RFC2045 line-length limitation must not be enforced. In current implementation it is only possible to remove all line breaks in the document using org.apache.xml.security.ignoreLineBreaks, sacrificing human readability.

Also, using LF instead of CRLF may be desired in systems where verifying side does not expect escape sequences in base64Binary values.

---

I would be thankful for your comments and suggestions.