mssql: add support for a user supplied msghandler

As things stand, it is impossible to gain access to messages emitted
from the server from a stored proc. Add support in _mssql for providing
your own msghandler (which gets executed from pymssql's msghandler).
Merge branch 'master' into msghandler_support
Add tests and docs.
Try to make tests work on Python 3.x.
Merge pull request #139 from pymssql/msghandler_support

mssql: add support for a user supplied msghandler