JSON datatype support for Postgres

Fallback to `text` for MySQL and SQLite.
Merge branch 'master' into json

Conflicts:
	index.html
default to drain the pool on the process 'exit' event
cleaning up some of the building logic for comments
finishing changes for foreign key support, fixes #24
a few more tweaks to get the foreign key support just right
hasTable now returns a boolean rather than a failed promise, fixes #23
using datetime rather than timestamp for the 'timestamps' method
support for enum in postgresql, fixes #21
merging PR #20 from vjpr:json
version check on the current client
adding support for the uuid data type