Add support for datetime64 dtype

[astrofrog]

This adds support for having Numpy datetime64 components in Data objects, and also updates the Matplotlib viewers to properly support this and show date/time labels.

@rsangole - feel free to try out this branch if you are familiar with git! (otherwise this will be available in the developer conda package for glue shortly after this is merged). Just to clarify, this never actually worked properly before - what you saw before was that datetime columns got converted to strings and glue would then show it as a categorical component, but the points were not actually properly spaced in time.

NOTE: selections don't work yet, I'm working on fixing that

Remaining TODOs:

• Add test of making a viewer with datetime data and modifying the xmin/xmax limits
• Add test of saving/reloading session
• Add documentation about this
• Add changelog entry

Fixes #1490