[diagrams] Attributes

[Brent Yorgey]

I just added attributes using a rather cute idea I stole from xmonad.
Basically it allows any type at all to be used as an attribute,
through the magic of Data.Typeable. Backends can handle whatever
attribute types they choose, and ignore the others.  As a proof of
concept I've added stroke and fill color attributes, add added
handlers for them to the cairo backend.  But if anyone's itching to do
some hacking feel free to add other attributes -- porting the
attributes that were supported by the old diagrams library would be a
good start.  If no one's itching to do some hacking no worries, I'll
get around to it eventually (or have my students do it =).

-Brent