Rework autocommit.

Make it use default database-layer feature mode instead of explicit
commits on the way.
Undo cancellation of rollback/begin of transaction in autocommit()
Add some tests for autocommit feature
Fix broken tests. Change parameter order. Tweak docstrings.
tests: Python3 compatibility. Proper asserts and failures.
Merge pull request #267 from pymssql/pr210

PR #210 ready for merge.