Added cpack configuration, and Mac Bundle package.

This PR enables CPack support in our CMake file. For the moment, I could only test Mac support and I enabled it in its "Mac Bundle" version, but following very simple instructions it is possible to generate debian packages, rpm packages, and so on. This can be changed with the CMake variable `CPACK_GENERATOR`. I have added all configurations for mac bundle package, and for now that is the default.  

This PR adds a "package" target. On a Mac, this target generates a disk image file containing a deal.II.app bundle (a directory with special structure, that Mac OS X recognizes as an application) which can be distributed without requiring the user to install anything. Just drag the "deal.II.app" in the /Applications folder, and the user will have a "deal.II" application (with our cool icon) which fires up a terminal where all environment variables are correctly set for deal.II to function "out of the box".
... depending on another cached variable

will lead to nasty cache invalidation problems. I.e. <code>cmake</code> will have to configure multiple times :-/

What about reducing this by defining <code>DEAL_II_CPACK_EXTERNAL_LIBS_TREE</code> as a cached option in  <code>setup_cached_variables.cmake</code> under <code>Miscellaneous options</code> with the empty string "" as default?

And just guard with
```
IF(NOT "${DEAL_II_CPACK_EXTERNAL_LIBS_TREE} STREQUAL "")
  ...
ENDIF()
```