JSON datatype support for Postgres

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

Conflicts:
	index.html
foreign key support, #24
missing semicolon
allow empty value passed to transacting function
fixing foreign key issue #24
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
fixing test output for travis
removing travis email notifications
add 'returning' method, to specify return value, fixes #18
don't null the transaction until it's completed flushing