Further changes for better library support.

Add the qmake variables MNE_INCLUDE_DIR and MNE_LIBRARY_DIR to configure the build process. It is now possible to generate a "mne library" with the related header files, which can be used in external applications.
To prevent naming conflicts on file system level the include folders should get a library prefix like "mne": mne or mneCore, mneFiff, mneMath and so on.