Add `neobundle#has_fresh_cache`, which compares it to $MYVIMRC

This allows for easy cache busting, if you have edited your vimrc.

I have not changed the example code for `has_cache`, since you might often
change things not related to the cache, but it's useful to have it as a
helper method.
Merge pull request #273 from blueyed/add-has_fresh_cache

Add `neobundle#has_fresh_cache`, which compares it to $MYVIMRC