improved fontsize behavior

I just made a change to cvs that users should be aware of. The font.size
setting in your matplotlibrc file previously defaulted to "medium", which
meant 12pt. I changed it such that font.size now expects an actual number, in
order to allow you, the user, to change this default size from 12.0 to
whatever you like.

All other font sizes, like for tick labels and axes labels, can be given in
relative sizes ("small", "large", etc.). If you find that on the whole, the
text in your figure is too small, you can change font.size and it will scale
all the relative font sizes accordingly.

I hope this is not an inconvenience to anyone.