Descent nightly builds

[Ary Borenszweig]

Since the timespan between each release of Descent is big, I decided to 
make some kind of "nightly builds", so that if you report a bug and it 
is fixed in trunk, you can verify that it's working right away.

You can also test new features that way instead of having to wait for 
the next release.

You can read the full explanation in here: 
http://dsource.org/projects/descent