Refactored the Opusfile cmake rules into own module

To properly find Opusfile on OS X (and possibly other systems too) a
hint had to be added to find the include directory of Opusfile. To keep
the CMakeLists.txt file clean, a new cmake module was created for
finding the Opusfile include directories and libraries.
buildsystem: prettified findopusfile
Merge branch 'master' of github.com:SFTtech/openage