Implement autoLoad={false} in Select.Async

[f0urfingeredfish]

In the current documentation autoLoad={false} is listed as an option to stop the Async Select from loading when the component mounts.
https://github.com/JedWatson/react-select#async-options

This isn't currently implemented.
Look at:
https://github.com/JedWatson/react-select/blob/master/src/Async.js#L82

FIX:
Add autoLoad as an option with a default of true.
Check autoLoad on componentDidMount to determine if it should fetch.

[bvaughn]

FYI I am working on a refactor for Async that should address this issue. (See PR #1226)

[bvaughn]

Resolved via PR #1226.

Look for an RC2 to come out (hopefully) sometime today with this change.