Added a find_all method to the RcParams dictionary.

I find I want to search for RcParams by keyword quite frequently.

Now you can do:

```
>>> import matplotlib
>>> print '\n'.join(matplotlib.rcParams.find_all('font').keys())
font.cursive
font.family
font.fantasy
font.monospace
font.sans-serif
font.serif
font.size
font.stretch
font.style
font.variant
font.weight
legend.fontsize
mathtext.fontset
pdf.fonttype
pdf.use14corefonts
pgf.rcfonts
ps.fonttype
svg.fonttype

```