Added live-set-default-font

live-set-default-font changes the default font and updates all windows
if emacs is running in XWindows, Win32, or NS drawing systems.

live-set-default-darwin-font calls live-set-default-font only if running
in drawin.

Menlo is only set on drawin.
Merge pull request #94 from rothmichaels/darwin-font

Added live-set-default-font