Add browser build

[curran]

Closes #173

[curran]

This PR adds a browser build by leveraging browserify-shim, and the --standalone argument of Browserify. The output bundle expects that React and ReactDOM globals are defined, but bundles the classnames package as it's so small.

This PR also adds a minified version of the browser build.

This change will make the library usable in a browser environment.

By the way, I'm not sure why there are minor changes in dist/index.js. I'd guess it's a Babel version bump or something.

[curran]

FWIW, in the mean time I've published the fork with the browser build here https://github.com/datavis-tech/react-dropdown-browser .