Added live-set-default-font

Updated the previous font submitting code with doc strings and adding ability to set default fonts on systems other than darwin.

- 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.

Where would you like me to put this in your doc pages? Under the FAQ?