Updated README

Signed-off-by: Vinzenz Feenstra <vinzenz.feenstra@gmail.com>
Bugfix in transaction class

From documentation: "END TRANSACTION is an alias for COMMIT." (http://www.sqlite.org/lang_transaction.html) Removed commit()+end() commands on destructor. Also added a flag in order to avoid commit when no transactions are open.
Added possibility to define the transaction type on construction/begin.