Exposing/documenting CFTimeIndex as public API

[shoyer]

I think we expose CFTimeIndex as public API, as either xarray.CFTimeIndex or xarray.coding.CFTimeIndex. Otherwise the only way to make this thing is by putting cftime datetime objects into an xarray object, and/or users will start importing it from xarray.coding.cftimeindex.

We should also add it to the API docs, for which it will need at least a minimally informative docstring, too.

CC @spencerkclark @jhamman

[shoyer]

Potentially it would also make sense to add a specialized constructor that is an alias for wrapping cftime.num2date in a CFTimeIndex, but we can save that for later.

[jhamman]

I'd like to see a utility that adds a calendar argument to pandas' date_range. When non-standard calendars are used, it would return a CFTimeIndex. Since that isn't going to happen right away, let's just expose the index class now.

[jhamman]

I'm working on the doc update now.

[spencerkclark]

Great, thanks @jhamman