Add a static .prefetch() method to async! wrapper

[developit]

What kind of change does this PR introduce?

Introduces a new static .prefetch() method on AsyncComponent (the wrapper component returned by async! imports:

import Foo from 'async!./foo';

// Perhaps we know for sure we'll need Foo soon:
Foo.prefetch();

// later on, this render can then happen synchronously:
render(<Foo />);

This can be useful for loading related routes.

Did you add tests for your changes?

Nope.

Summary

It is sometimes useful to prefetch lazy chunks when you know they'll be needed soon. This is a pretty tiny and unobtrusive way to do it.

Does this PR introduce a breaking change?

Nope.

[lukeed]

You almost exceeded your byte limit on this one.

[lukeed]

Our tests be like

[developit]

Test errors look like a cert generation issue.

[reznord]

Yup, can we fix the tests cert issue as well? (For making travis happy)