Restore optimized marker rendering in special cases.
Add test cases that provide basic code coverage and simple rendering validation.
Update CHANGES.txt
Merge branch 'master' into fix/restore_optimized_scatterplot

Conflicts:
	CHANGES.txt
Trivial clean up of unit test.
PEP8 fixes to test.
Use isinstance instead of __class__.
Merge branch 'fix/restore_optimized_scatterplot' of https://github.com/enthought/chaco into fix/restore_optimized_scatterplot

Conflicts:
	chaco/tests/scatterplot_renderers_test_case.py
Fix one more isinstance case.
Merge branch 'master' into fix/restore_optimized_scatterplot

Conflicts:
	CHANGES.txt
Merge branch 'master' into fix/restore_optimized_scatterplot
Merge pull request #178 from enthought/fix/restore_optimized_scatterplot

Restore optimized scatterplots