Added feature to disable implicit and enable explict transaction

The plugins to persistence of VRaptor every open a implict transaction.
It is good, but in the some cases generate database locks turning the application very slow and with long pauses, specially when the transaction is slow. Ex.: Download large files from database.

This PR is to add feature to disable by properties the implicit transaction and enable it by a annotation.

On [this project](https://github.com/garcia-jj/vraptor-plugin-hibernate4/pull/13) the feature is disabled with annotation, but I think that is not good solution because there are more read operations that insert, update and delete.

This feature not change the current behavior.
I think that this feature should be applyed on the JPA Plugin too.