Support setting DataArray.name in-place
No longer support setting DataArray.variable in-place

I can't think of a legitimate use for this.
Support modifying individual DataArray.coordinates in-place
Support renaming a dataset inplace

This will be helpful to adding support for in-place modification
of DataArray coordinates and dimensions.
Properly convert Index and Coordinate into DataArray
Support setting DataArray dimensions and coordinates outside constructor

Fixes #180
Support for setting Dataset coordinates

Also added more tests for the various edge cases when setting
DataArrayCoordinates.
Another test case for DataArray.name = "y"
Test case for setting Coordinate.name
Doc string for DataArray._set_new_dataset
Fix two old references to xray.Index
Merge pull request #192 from shoyer/modify-in-place

Enhanced support for modifying Dataset & DataArray properties in place