Split project in multiple modules.

Hi,
First of all, thanks for the great job you're doing.
As we are currently only interested in the play-json column type for a prototype project, we did not want dependencies on json4s, joda-time, ...
To solve this, we moved some files and added some modules, one per library dependency. 
we only moved source files, not tests; those one stay in the master slick-pg project that depends on all modules.
There is no changes in the code, only some moves that can be useful for other.