Allow shallow `git clone`

Added the ability to perform shallow git clones. This helps reduces the amount of code downloaded to only the current version. In some cases the difference can be significant, for example when downloading Valloric/YouCompleteMe the difference is about 30MB.

Two options were added to control this functionality:

* g:neobundle_git_shallow_clone
* g:neobundle_git_clone_depth

g:neobundle_git_shallow_clone enables/disables the inclusion of a --depth=<g:neobundle_git_clone_depth> parameter to the `git clone` call.