Add React components for each format type

This adds React components for each format type, e.g. `Number`, `Date`, `Message`, et al. These components have been added in a way that's backwards compatible with v1.0.0.

The new way to use this package is to reference the new `ReactIntl` global in the browser, or use `var ReactIntl = require('react-intl');` in Node.js.

Instead of the Mixin being the default export, the `ReactIntl` module now contains a named export for each component, the mixin, and the function to add locale data. Completely they are:

```js
// Component Mixin:
ReactIntl.Mixin

// Components: (e.g., <Number style="percent">{0.9}</Number>)
ReactIntl.Date
ReactIntl.Time
ReactIntl.Relative
ReactIntl.Number
ReactIntl.Message

// Function to add locale data:
ReactIntl.__addLocaleData
```

**Note:** This should be safe to release as `v1.1.0` as all back-compat is maintained.

Closes #23

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### Todos

- [ ] More testing!
- [ ] Try on FormatJS site.
- [ ] See if `O.dP` is okay to use in browser.
- [ ] Update README.
- [ ] Update docs on FormatJS site.

This needs to be tested. By looking at the docs, it seems like it will do what we want — `throw` if there's more than one child.