Packaging on Mac OS

This PR now integrates packaging on Mac OS. This also fixes #34.

When the `package`-target is build a .dmg-file is created which contains a nice Drag and Drop installer (simply move the ogs-gui icon somewhere, e.g. into the Applications folder). This package is redistributable on the same Mac OS X version (I tested it on 10.9 systems). Packaging is now enabled by default, simply type `make package` or build the `PACKAGE`-target in Visual Studio. Further additions / changes:

- Removed option `OGS_PACKAGING` and `OGS_PACKAGING_ZIP`
- Added option `OGS_PACKAGING_NSIS` for Windows installers
- Small fix for including Eigen on Windows
- Dumpbin should now be automatically found (a tool necessary for Windows packaging)

One important note when you want to package on a Mac and redistribute the file: if you have installed your library dependencies (such as Boost, Qt, ...) via Homebrew they get optimized for your current cpu architecture and so the OGS executable then also only runs on that architecture. But you can install libraries unoptimized via the `--build-bottle` parameter (see also [this SO question](http://stackoverflow.com/questions/20114920/can-an-exec-bad-instruction-sigill-on-dyld-my-fault)):

    brew install --build-bottle qt

Just in case someone wonders about the `.DS_Store`-file: this file stores the visual layout of the Mac installer. Unfortunately it is a binary file and cannot be edited easily...