Update Unite, Multiple Cursors, GitGutter, Color!

Major update to vimrc

Add several sources to Unite.vim (neomru, help, junkfile, session). Add mappings
for each new source and update old mappings for other sources.  Fix clash
between Unite and Surround.  <C-G>S mapping from surround clashed with <C-G> in
insert mode.

Replace multiple cursors with kris89 fork.  Solves conflict with Vim-Sneak where
Sneak "s" mapping was expanded while in multiple-cursor-insert mode.

Fix GitGutter mappings to use the Ex command instead of the <Plug> mapping.  The
mapping wasn't working for some reason.

Update ColorToggle to use new SetBackgroundTheme function.  Done so that updates
to the background theme can happen in one place.  Was currently in several due
to vimrc setting theme on startup.

Change right prompt and editor command for vimshell.

Update Bundle list to reflect obsolete bundles after Unite update.

Replace calls to !mkdir with calls to internal mkdir function.  Increase
portability of vimrc.