[NOP] added xvfb to travis execution to allow gui related stuff

 * since doc is now part of ALL and doc needs a GUI we need travis to run with an X Server (xvfb)
[NOP] removed old, now obsolete installation cmake files
[BUILD] added installation of header alongside openms

 * note that this doesn't necessarily allows compiling against the installed
   headers, since the dependencies may not be visible (e.g., private contrib)
[BUILD] made install directories customisable
[DOC] updated documentation w.r.t. to the use CMAKE_INSTALL_PREFIX

 * some general documentation improvements (e.g., replaced svn with git on the touched pages)
[BUILD] fixed mac dmg package generation w.r.t. to new install scheme

 * moved general package defs to separate file