Merge branch 'master' of https://github.com/lwhite1/tablesaw to get R…

…eadme.MD changes

and pom changes synched.

removed depenendency on local directory structure from pom. Will need…

… to fix javadoc build now.

Finished cleanup on descriptive statistics for all numeric column types

fixed append bug (issue jtablesaw#62)

Replaced RoaringBitmap where it was exposed in the API with an Interf…

…ace called Selection.

Added Selection and a RoaringBitmap-backed implementation called BitmapBackedSelection
The intent is to provide greater flexibility going forward

fixed method naming for countMissing()

cleanup naming of columns derived from viewgroups during reduce opera…

…tion

tests for float column. mostly smoke, a few functional

minor tweaks; cleaning up Table

Update README.md

fixed broken link