Run unit tests with execinbg: true by default
Include columns with 'null' values in result set

This fix updates the plugin so that it includes columns with a 'null'
value in the result set. This mirrors the behavior of the HTML5 Web SQL
API.
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_