ENH better error message when wrong cmap name.

Now prints all possible choices
Re-enabled PEP8 test, closing #2443.
add failing test: savefig(StringIO(), format="ps")
Merge pull request #2491 from pelson/pep8_re-enable

Re-enabled PEP8 test, closing #2443.
Merge pull request #2485 from NelleV/error_message

ENH better error message when wrong cmap name.
add new ps backend test file in `__init__.py`
Fix writing to all manner of StringIO-like objects in PS backend.
Merge pull request #2512 from mdboom/ps_StringIO

Fix saving to in-memory file-like objects in Postscript backend
Fix encoding issues when font name contains non-ascii characters
Minor doc fixes
Merge pull request #2523 from mdboom/ps-unicode-error

Unicode issue in EPS output when using custom font
Merge pull request #2526 from thenovices/fix-typo

Minor doc fixes