Clone repository recursively, submodule'ed themes

Git submodules are great to keep track of themes without having to manage them all in one repository.

But cloning the main repository only gets you empty folders for the submodules; an additional init is needed.

It's easier and more convenient to just clone the main repository recursively, in order to pull in all themes.
Merge pull request #94 from daniel-j-h/patch-1

Clone repository recursively, submodule'ed themes