Adding countouring c library
Things to do...
Initial isocontour passing tests
Further contours (and related tests)
Renamed variables to reflect new tranform options.
Integrated testing changes and modified colors
Added 'points' flag
Fixes error in ID if passed grid is empty
Fixing missing reference to isocontour in setup.py
Updated spread and fixed Seq creation error
ISO contours now return just contour (not envelope)
Renamed 'isocontour' to contour and contour to _cntr
Some documentation updates
Documentation
Merge branch 'feature/contour' of https://github.com/ContinuumIO/abstract_rendering into feature/contour
Updated contour splits and tests.
Allow failures tag
Updated version number
Merge branch 'feature/contour'