Vundle migration

This should upgrade Vim's plugin system to vundle.
I've tested as much as I can but it would be nice if somebody could give it a go (backup your .yadr dir first!)

Ideally all you've got to do is `rake update` and that should perform the following actions:
1. move vim/bundle to vim/bundle.old
2. remove all submodules from the git cache
3. clone vundle and install it under vim/bundle/vundle
4. run BundleInstall and install all the plugins definited in vim/vundles.vim (which is, by default, the same list of plugins currenty on yadr's master minus csaprox. See commit history for the reasons behind this)

`rake update` detects the need for a migration by checking if pathogen is present in the bundle directory.