Add `fmt_r` and `fmt_theta` methods to polar axes

The example here works in Cartesian coordinates:

http://matplotlib.org/examples/pylab_examples/coords_report.html

but if you change 

`subplots()`

to

`subplots(subplot_kw={'polar':True})`

Then the `millions()` function is never even called.

( no response on mailing list: http://matplotlib.1069221.n5.nabble.com/fmt-xdata-fmt-ydata-on-polar-plot-td45838.html )
