Make SQL keywords case-insensitive

This is a bit of a hack that allows all variations of a keyword, but it still seems to produce valid error messages and such.

Author: Matei Zaharia <matei@databricks.com>

Closes #193 from mateiz/case-insensitive-sql and squashes the following commits:

0ee4ace [Matei Zaharia] Removed unnecessary `+ ""`
e3ed773 [Matei Zaharia] Make SQL keywords case-insensitive
Add asCode function for dumping raw tree representations.

 Intended only for use by Catalyst developers.

Author: Michael Armbrust <michael@databricks.com>

Closes #200 from marmbrus/asCode and squashes the following commits:

7e8c1d9 [Michael Armbrust] Add asCode function for dumping raw tree representations.  Intended only for use by Catalyst developers.