Add BufKill and Command-W, fix up shift write/quit commands

Fix:
 * The cmap W/Q combinations to use command! as it interfered with
   Command-W

Add:
 * BufKill vim plugin
 * Command-W vim plugin
 * gvimrc file for Command-W MacVim configuration
 * gvimrc file to Rakefile for symlinking
Fix up Rakefile's install task to work correctly for a dry_run

Fix:
 * The install task's dry_run was evaluating the string as a bool.
   Convert the param to be a string.