Added support for HP Vertica (tested on HP Vertica 6.5 and 7).

I added support for HP Vertica, issue #344, and ran some basic tests. HP Vertica uses a similar syntax and built-in functions to PostgreSQL, but it deviates in some areas (v_catalog instead of information_schema, BEGIN/END instead of $$) and has a few unsupported features (no indexes). 

I'm not aware of a published list of differences between PostgreSQL and Vertica. However, I still based the tests on the PostgreSQL database support, with modifications where required.