[FEATURE,BUILD] fix install target

This series of commits fixes the install target. OpenMS now supports the CMAKE_INSTALL_PREFIX variable to install the library, binaries, share, and documentation. It also install the headers but this is not yet fully functional since we still lack a fully functional export of CMake targets and the includes paths (need to be targeted in near future).

To support this I needed to customise the meaning of `OPENMS_DATA_PATH`. Before, the build system set this either to `<src>/share` or `INSTALL_PREFIX/share`. Now we support both paths (`<src>/share` gets searched first). Same holds for `OPENMS_DOC_PATH`.

This pull request also

 * fixes the dmg package generation for mac os x. Additionally 
 * I added the doc target to ALL such that it is build before install
 * travis-ci now uses xvfb to also run the GUI tests (and build the documentation)
 * further cleans up the build system

Fixes #539 and #701