Added entry point for browser bundles

The code is the same as in the `index.js` file, but without the `require('./lib/locales')` line, which caused the inclusion of the locale data on the bundle.
Yes, `global` references `window` when bundled for browser (at least browserify, I'm guessing webpack does the same, I'll confirm later).

BTW, I haven't been able to solve nicely the fact that the bundle includes `ReactIntlMixin`, so it must be evaluated before the `<script>` tag that includes data for other locales, since doing so means that you shouldn't format something because it will be in the default locale. And, if you put the `<script>` that references the locale resources before the bundle, it doesn't work because `ReactIntlMixin` is not defined. I ended up doing an ugly hack by wrapping the `ReactIntlMixin` locale specific data in function at build time, so I can call it from inside the bundle once `ReactIntlMixin` is exported to the global scope, but it is still an ugly hack :).