Vim: Use plugin vim-powerline

Add bundle 'powerline' for vim, and activate the plugin.
If powerline does not present, an older statusline is used instead.

Powerline-specific fonts are required to display powerline correctly,
which are available at: https://github.com/Lokaltog/powerline-fonts

Moreover, latest vim version is required (possibly 7.3.807+) for
not suffering from a bug where autocomplete menu doesn't show
when powerline is enabled.