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

Preceded by: #586
Followed by: some testing tools (not ready)

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.