Dont re-create EngineResolver during Engine registration

This fixes the problem when you are trying to use multiple EngineBridges.

Also, it makes stuff much cleaner as you are now re-using the already created `EngineResolver`.

There was also another [PullRequest][1] sent to [XsltBridge][2] as those two bridges were the ones I notices clashing.

 [1]: https://github.com/iwyg/xsltbridge/pull/3
 [2]: https://github.com/iwyg/xsltbridge