[dwm] Keys with prefix

I've written a pull request to allow prefixed keys.

With this pull request you can bind ``Java 7-t k`` (where ``Java 7-t`` is your prefix),
for instance, to kill window or ``Java 7-t 1`` to ``view("1")``. You can
change the prefix by editing the line that contains the ``setprefix``
function in the config file.

I've written this pull request because:

* Some of dwm's default keys conflict with emacs keybinding
* Some emacs users, including me, don't like to push three buttons at
  the same time very oftern
* If ``? x`` (? is the prefix and x is a tag) is bound to view("x"),
  letters can be a tag like didropboxs.
* Prefixed and normal keys can coexist

I didn't know dwm's policy for submitting pull requestes; so I've attached
the outcome of ``hg export``.

Regards,
--Ali