Rename xray.Coordinate to xray.Index

Fixes #142.
Rename indexed, labeled, select to isel, sel, select_vars

Also renamed unselect to drop_vars.

The logic behind these changes is that it would be really nice
to have a short method for indexing by dimension name, since this
is one of the most common operations one might do. Also, prefacing
an indexing method with "i" nicely mirrors how pandas names its
indexing methods/properties -- now it's clear that "sel" and "isel"
are variants of each other.

Fixes #148.
Merge pull request #161 from shoyer/renamings

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