Added ALTER TABLE support (add/drop column)

@brianc: Note that I didn't add RENAME support (for renaming columns or renaming tables) because the syntax is a little different and doesn't support multiple actions in the same statement like add/drop columns do (see: http://www.postgresql.org/docs/9.2/static/sql-altertable.html)

Also, I hung the add/drop column nodes on the alter node instead of adding them directly to the query node because I needed to comma-delimit them in `visitAlter()`. Perhaps there's a better way to deal with that...