Add GPL license support for new gems using the --license cli parameter.

I've added a --license switch to the gem command since I prefer to use the GPL instead of the MIT license.

When running `bundle gem GEM --license GPL` it creates a COPYING.txt file with the GPL license and sets the license in the gemspec accordingly.

I can add more licenses, if this gets merged (LGPL, AGPL, BSD?)

Thanks,
Andrei