Rename "Coordinate", "labeled" and "indexed"

Fixes #142 
Fixes #148

All existing code should still work but issue a `FutureWarning` if any of the old names are used.

Full list of updates:

Old | New
----- | ------
`Coordinate` | `Index`
`coordinates` | `indexes`
`noncoordinates` | `nonindexes`
`indexed` | `isel`
`labeled` | `sel`
`select` | `select_vars`
`unselect` | `drop_vars`

Most of these are both `Dataset` and `DataArray` methods/properties.