Add a pandas option to PointCloudReader

Like with numpy, this will gracefully fall back if pandas is not available.  If
it is, it will return a Panel with the same axes, but fortunately the genes can
be labelled.
Fix time bug in PointCloudReader

Although each time point was internally consistent, they were being given in
the wrong order in the final output array.  Stupid me for not sorting the set!
Merge branch 'master' of github.com:petercombs/EisenLab-Code

Conflicts:
	PointClouds.py