Support blobs as Base64 Strings

Pulled from code by Steve Davis,
https://github.com/stdavis/OfflineMbTiles/blob/master/OfflineTilesProof/
SQLitePlugin.m#L181
Base64 blob implementation using public domain libb64

Embedded the libb64 encoding algorithm as static class methods within
SQLitePlugin.  Two typedefs added to the global namespace, prefixed
with LLIB64_
Transitioned libb64 from class methods to static c functions in .m

Retained class method
    (id) getBlobAsBase64String:withlength
This could also be eliminated if desired.
Merge pull request #1 from tbbdeveloper/Blobs-as-Base64-Strings

Support blobs as Base64 Strings