Compatibility with Psycopg 2.5

Hello,

this branch brings psycopg2-cffi at the same feature level of psycopg 2.5.

The psycopg 2.5 test suite has been dropped in and the code hammered until all tests passed. A couple of corrections to the tests and the pure part of the lib have also been applied upstream.

Also fixed interaction with older Postgres versions: tested with PG 7.4-9.2.