Fixes for tests under PG and EXISTS/NOT EXISTS

[Peter Brant]

Hi Max,

It looks the test suite is broken against Postgres on master. The two
problems were:
- A constraint name needs to be an unqualified name on Postgres.
Fix is here https://github.com/max-l/Squeryl/pull/108
- The HAVING test was generating SQL that didn't work on Postgres.
Fix is here https://github.com/max-l/Squeryl/pull/109

I also added support for EXISTS/NOT EXISTS. Patch is here
https://github.com/max-l/Squeryl/pull/110 Let me know if anything
should be done differently. Unfortunately it's only marginally useful
because Squeryl doesn't support correlated subqueries (yet). I'll
send a separate email about that though.

Regarding feeding you patches -- are issue/pull requests on github
still the way to go? or would you prefer something in Assembla?

Thanks,

Pete