Remove promise duplication.

[monder]

Fixes #116

It may not seems obvious with all stuff happening with promises in react-router but methods do not mutate those.
e.g.

let a = new Promise(...)
let b = a.then(...)
let c = a.then(...)

b and c are completely different. And if the exception will be thrown in a there will be 2 unhanded ones.

We should either always handle promises without breaking the chain, or wrap every split with a catch block. This PR implements the 1st option.

[ericclemmons]

@monder Good catch! The examples still work as well?

[monder]

The example for react-0.14 works just as before.
Tests pass.
It works on my app with react 15 as well.

[ekmartin]

Thanks @monder!

[ekmartin]

Would be nice if you could publish a new version when you have time @ericclemmons 😃

[monder]

Just published the version 3.0.3

[ekmartin]

Great, thanks! Works like a charm.

[ericclemmons]

@monder What's your Twitter name? I want to broadcast the work you're doing :)