Moved some rest-dispatch code to a shared project. Update carstore

Preceded by: #586
Followed by: #590 

Whilst the previous commits are backward compatible, this one is breaking-change. Mostly, some classes were moved between client <-> shared packages, so nothing too big. The Rest-Dispatch is flagged experimental, so I'm not gonna go through the hassle of documenting the breaking changes for now. The current structure is final/near-final so the flag will be removed soon!

**Summary of the changes**:
- Created a gwtp-dispatch-rest-shared module. The goal is to import only the necessary classes on the server's classpath and reuse your resource interface.
- Updated the carstore to reuse the resource interface. This way the annotations are declared only once on the interface. There's a gotcha though, [see this reference](https://jsr311.java.net/nonav/releases/1.1/spec/spec3.html#x3-330003.6) for more info.
- Couple code-style and namings to the carstore fields / types.