Merge lp://staging/~cjwatson/storm/psycopg-2.5 into lp://staging/storm

Use ABCMeta for exception base class injection

This requires Python >= 2.6 (which is unlikely to be a problem these days!),
and adds compatibility with psycopg2 >= 2.5.

Use ABCMeta for exception base class injection.

This lets us be compatible with psycopg2 >= 2.5, where Error and subclasses are implemented in a C extension so can't have their __bases__ attribute patched.

It does introduce a potential obstacle to porting to Python 3 due to a Python bug noted in a comment. Other possible approaches include converting StormError et al into tuples (but that would be an API break if anyone was using them for any purpose other than isinstance/issubclass/exception-handling) or re-raising all database exceptions as Storm exceptions (rather intrusive and possibly error-prone). Since Storm hasn't yet been ported to Python 3 anyway, this doesn't seem like a showstopper, and I think using ABCMeta is a pragmatic approach for the time being.