added 'savefig.transparent' rcParams

Closes issue #2388
minor tweak to `pyplot.savefig` to ensure that if `transparent=True`
is passed to `savefig`, the patch colors are restored on the screen
after saving.
comment tweak

As suggested by @dmcdougall
Added entry to whats_new.rst
Merge pull request #2390 from tacaswell/transparent_rcparams

Transparent rcparams