OGM-279 Enable checkstyle on OGM

https://hibernate.atlassian.net/browse/OGM-279

I've added several rules that are compatible with the formatter we use in eclipse.
You can check the meaning of the different rules I've used in the checkstyle.xml on this page: http://checkstyle.sourceforge.net/availablechecks.html

Let me know if you want to add more rules :)

I know that this kind of pull span over a lot of files but I think the sooner we start to use it the better.

Cheers