A few small fixes/additions to the postgres block store

This pull request adds some functionality to the postgres block store that I thought others would find useful:
1. Adds a constructor so you can pass in the jdbc url instead of only the hostname and database name.  
2. Adds the ability to use a schema to store the tables in - by default they were being stored in the 'public' schema, but users may want to put it in other schemas.
3. Fixes an exception in close() when it was attempting to rollback a connection where autocommit was true.

Also adds a test to the PostgresFullPrunedBlockChainTest to make sure it works with a schema name set.