Add custom target in order to create tarball

[Olivier Duchateau]

After talk, with Sam Thomas [1], patch provides custom command, in order to create tarball for next releases.

How to use it:

mkdir ../build
cd ../build
cmake ../tomatoapp
make dist

Another method, is to use bzr export [2].

[1] https://plus.google.com/+LuizAugustoMorais/posts/Jwbb5mJwQmS?cfem=1
[2] http://doc.bazaar.canonical.com/latest/en/user-reference/export-help.html