OGM-609 Adding performance test suite

@DavideD, @emmanuelbernard: I'm not yet completely done, but can you check it out and let me know what you think. To run the suite, do the following:

* mvn generate-sources -pl performance (generates the JMH benchmark code)
* configure the MongoDB host in `native-settings.properties` and `persistence.xml`, respectively
* run `org.hibernate.ogm.perftest.Launcher`, e.g. from your IDE; optionally adapt the `include()` call to match a subset of benchmark classes/methods only

Could you run the benchmark and let me know whether you think it's sound, test cases should be added etc. Personally I think the resulting numbers are ok-ish, at least not catastrophically bad from what I can say. I'm planning to add a test case for assocations.
Good idea, made it an option.

Note that I envision this to be invoked from the IDE most of the time (at least for now), so I didn't add options for the other stuff as you can easily adapt the includes, iterations etc. in the editor. But adding this option makes it runnable out of the box on more systems which is a good thing.