Feature/enable all packages and pdelab test macro

The branch uses the dune_enable_all_packages() feature for dune-pdelab.

To be compatible with cmake 2.8, the library libdunepdelab now has to be created in the toplevel CMakeLIsts.txt. From cmake 3.1 on, one would use the MODULE_LIBRARIES arguemtn of dnue_enable_all_packages() to avoid this.

Because I had to touch all tests anyway, I have combined this change with the introduction of a macro pdelab_add_test which makes adding any test a oneliner.

There is one remaining issue from disabling the build-on-make-test black magic: The injected include(BuildTests.cmake) wont go away as long as test_dep() from dune-common/cmake/modules/DuneTestMacros.cmake globs for directories called "test" and injects something there (check dune-common/cmake/scripts/FindFiles.cmake for the actual globbing). It delivers an error, but it is not fatal. I am intrigued to say, we dont work around this until the upstream fix.

Fixes #6 (closed) and #7 (closed).