reworked _autocommit_ to use default database-layer autocommit mode

Previous implementation of autocommit used kinda-automatically-created-and-automatically-commited-transaction. However, MSSQL has another internal way of doing autocommit: http://msdn.microsoft.com/en-us/library/ms187878(v=sql.105).aspx

So the change is made in favor of bringing things more to a standart way to do and to allow various DDL statements to be run through pymssql, including 'drop database' and 'restore database'. They can't be run inside a transaction so one has to use autocommit=True in their connection to allow these statements to get executed.