[BUILD] minor adaptions to the build system

 * removing the test_build related infos from the targets target
 * updated the pyOpenMS cmake to be a bit more OpenMS like
[DOC] removed test_build target from the documentation

 * incl. further cleanups related to the repo restructuring
Merge pull request #662 from aiche/feature/cmake-clean-ups

CMake and docu cleanups w.r.t. to the new layout