matplotlib Code

Minor changes.

Update list of supported backends.

Update list of supported math symbols.

Update information about the backends (Ps and Pdf were not mentioned.)

Preliminary version of "adding new scales and projections" document.
(In reST, since that appears to be where mpl documentation is
heading.)

Improved documentation and efficiency...

Fix log scaling of polar plots.

Fix bug in calculating minpos.

Don't allocate the clipping path buffers unless we need them.

Minor speed improvement