vimrc: allow undo of Ctrl-u and Ctrl-w in insert mode

Ensure that my changes can be undone if I accidentally press Ctrl-u or
Ctrl-w in insert mode.
Add Gundo.vim plugin

Shows a visual representation of the current file's undo tree. Makes it
easier to see what you did and when, and roll back to a previous version.

I tried installing undotree.vim
(http://www.vim.org/scripts/script.php?script_id=4177), a pure
Vimscript plugin that's similar, but it requires vim 7.3 with patch005
applied which I don't have, and I'm not crazy about compiling a new vim.
vimrc: add <leader>gu mapping to toggle Gundo
README: add Gundo to plugin list