Introduce a :nulldb_postgres adapter for providing postgres features.

This is not quite ready for merge, but I want to put it up for discussion. Currently nulldb blows up if you use any of the fancy new postgres features that Rails 4 rolls out. I set about getting them working an this is my approach: creating a minimal second adapter which extends the first, and adding some extension points to enable them to operate against one another.

The main concern I have at this point is not knowing how to reliable identify the original adapter type of the database, before it was nullified. I have it working in nulldb_rspec, but not in nulldb/core.

Any thoughts/advice would be appreciated.