Add Frame.fill constructor.
WIP: Blah.
Add some more helper methods to Series.
Add new stats package with ops.

The only ops currently supported are `summary` on Frame and Series
types.
WIP: Add tests for summary.
Merge remote-tracking branch 'upstream/topic/frame-fill-construction' into topic/a-few-methods
Add summary spec.
Merge branch 'topic/generic-with-date' into topic/a-few-methods
Add reduceFrameWithCol method.
Add zipMap back to Cell.
Mean returns NA when there are only NAs.
Merge pull request #19 from pellucidanalytics/topic/a-few-methods

Add various methods to frame, series, cell, etc.