add React support
add React support

Summary:
Add BabelJS option, React's official javascript compiler.

Previously, a static compiled version of BitGoJS was used in React related projects, partly due to React-Native specific bugs that have all been fixed recently.

With `npm run-script babelify`, users can install BitGoJS as a normal npm package, and still be able to run it successfully on top of React framework

Reviewers: benchan, ben

Reviewed By: ben

Subscribers: ben

Differential Revision: https://phabricator.bitgo.com/D1949

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS