Allow cffi version 0.6

Seems working ok: all the tests pass.
Ignore built library
Psycopg 2.5 test suite dropped in

FAILED (failures=16, errors=66, skipped=29)
Pure psycopg 2.5 modules dropped in

FAILED (failures=3, errors=89, skipped=39) (yep, worse :)
Added connection and cursor context managers implementation
cursor.close() idempotent
Fixed a few exception classes from code

Bad 55000 handling was making scroll raise the wrong exception
cursor.scrollable implemented

Also fixed broken cursor.scroll()
Fixed several imports dumbly pasted from -c
Added implementation of connection.cursor_factory
connection.close() is idempotent
Added pgerror, pgcode on Error base class
Populate pgerror and pgcode when the exception is raised
Fixed imports in pool module
Strip the severity trailing the exception message

Also fixed something else I'd messed up raising the exception.
Import the implementation under test in external scripts

Patch applied upstream.
Get the encoding in QuotedString from the connection
Typecasters are callable
Raise DataError on malformed array strings
Fixed QuoteString when prepare() is not called