Correctly invalidate connections on failure

_cursor does not check if the connection is still valid, meaning that
once a connection has been marked as invalid it will attempt to get an
sqlalchemy cursor on a broken connection and throw an exception. This
commit causes the connection to be validated when using the cursor,
triggering a new connection to be fetched from the pool if the previous
one has been marked as dead.
Merge pull request #13 from Jonty/master

Correctly invalidate connections on failure.