[SASL-48] Test case for the GSSAPI mechanism.
[SASL-45] Add a clean implementation of the GSSAPI mechanism as defined by RFC4752

This change also includes interoperability testing agains the JDK supplied versions.
[SASL-45] Correction to comma separated string splitting, also restore the two method forms for CallbackHandling to allow a mechanism to choose to handle the UnsupportedCallbackException itself.
[SASL-45] Add trace level logging to allow diagnostics relating to GSSAPI based authentication.
SASL Base64 coding/decoding fix.
Test case to test SaslBase64 class functions.
Utilizing new ByteStringBuilder append method.