Add support for store transactions

If backend adapter supports transactions (e.g. Virtuoso via ODBC)
handle transactions in backend, otherwise ignore.

The addMultipleStatements method now automatically starts a transaction
if no transaction was already started.

Also the backend method returns a boolean value whether addition was
successfull and the store class passes this value to caller.
Add transaction support for deleteMatchingStatements
Merge pull request #55 from AKSW/feature-store-transaction-support

Add support for store transactions