Merge underscore.string.fp

https://github.com/stoeffel/underscore.string.fp

Following the footsteps of Lodash v4.

Make this work

``` js
import {trim} from "underscore.string/fp";

var foo = trim("_")("_foo_");
expect(foo).toBe("foo");
```

Precompiled files for cherry-picking would be cool too

``` js
import trim from "underscore.string/fp/trim";
```

Maybe as a npm prepublish script.

[Like in Lodash vNext](https://github.com/lodash/lodash/issues/1704#issuecomment-170381265)