Recursively link files from nested submodules 

I may be mistaken but #108 is not corrected for me with submodules *inside* submodules, only the first level is linked.

I currently use homeshick (and I love it!) with bash to clone and link my dotfiles containing a submodule for dotvim using the pathogen plugin and thus another level of submodules. Only the dotfiles and the contained dotvim submodule are linked.

This PR contains a modified "link submodule files recursively" test and associated fixture demonstrating the problem and a minimal fix in the get_repo_files() function.