Make vim-projectlocal work with Powerline

When editing a new file, [Powerline](https://github.com/Lokaltog/powerline) loses its colours, as demonstrated in [this issue](https://github.com/Lokaltog/powerline/issues/230).

It seems that Powerline doesn't like it when you [resource .vimrc with autocmd](https://github.com/Lokaltog/powerline/issues/213#issuecomment-13507048), because the `ColorScheme` event [isn't fired](https://github.com/Lokaltog/powerline/issues/230#issuecomment-13689589).

The fix is to use [autocmd-nested](http://vimpluginloader.sourceforge.net/doc/autocmd.txt.html#autocmd-nested).