Conditional linking and adjusted tmux prefix

Remove files if they already exist before linking (for example, remove
~/.bashrc before linking to dotfiles version). Also changed tmux prefix
to C-a.
Added vimux to vimrc.vundle and fixed installer

Added vimux to the initial vimrc.vundle configuration. Also remove
entire ~/.vim directory before starting Vundle
Fixed Cask and shell installer

Remove ~/.cask directory (if it exists) before downloading and running
Cask. Fixed removal of .bashrc if it exists.