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.