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
use six.move for cStringIO
Merge pull request #2393 from davidovitch/use_stringIO_from_six

use six.move for cStringIO
support tight_bbox for pgf output, fixes #2342