API reorganization

Renamed "XArray" back to "Variable" and a bunch of associated names. Also
renamed the "data" attribute to "values" to match pandas (closes #97). Using
any of the old names should still work (for now) but raise a warning.
More API reorganization

Renamed: index -> as_index, indexed_by -> labeled, indexed_by -> indexed

Also, cleaned up the behavior of Dataset.select() as described in the README
(yes, that probably should have been a separate commit).
Renamed CoordVariable -> Coordinate
Added tests to increase test coverage percentage

We are up to 94% coverage.
Added more deprecated function
Merge pull request #100 from akleeman/rename-xarray

API reorganization. If you don't like the number of warnings you see with this, try the "stablish" branch from github.