Added support for file association handling

I have checked the library linked by [JanWerder](https://github.com/Torrunt/vimage/issues/17#issuecomment-46660669) and modified it a little so it does not require administrator privileges, only when writing to the registry.

With this done, and with meddling around in the swamp of WinAPI (... it's so... uck.), I think file association support should be implemented with this chage.

When started normally, the settings program will show file types that could be handled by vimage along with what they are handled by right now.
![No administrator rights](https://cloud.githubusercontent.com/assets/1969470/5527483/ef2406b8-89fe-11e4-9e53-328a53ed46d7.png)

Click on that little button, it will restart with Administrator rights. And then, the _Associate_ button becomes available which registers the file type for vimage.
![File type associated properly](https://cloud.githubusercontent.com/assets/1969470/5527501/1ec393e8-89ff-11e4-8963-2875b1fe9e45.png)

Settings the file icon (and the open command, of course):
![vimage-associated file in a folder](https://cloud.githubusercontent.com/assets/1969470/5527498/1b0a2bb8-89ff-11e4-8cc3-f811f9da520f.png)

(So this should fix issue #17 )