ES5 Support

[augustobmoura]

For support on old Node enviroments (I actually need this feature), I added a ES5 transpiled version and created a build npm script for running babel on the source code.

As far as I know there are just a few consts from ES2015, but for future incrementation I preferred to just add a hook to babel-cli. It's using the same .babelrc for the docs, I don't think we will have any problem with it anyways.

2 things:

• I can't run the tests on Windows, even in the master branch, if it's anything broken, please let me know.
• I used a relative path on the build npm script, I don't know if it's a Good Thing, just works fine to me, but you are free to change it.

[schmod]

@augusto-moura Thanks for the PR.

Which version of Node are you targeting? If any part of this plugin isn't compatible with Node v4, I'll gladly modify the source code to fix it.

Otherwise, I'm hesitant to include support for Node v0.10/0.12/5, as those versions are unsupported by the Node.JS foundation, and will be dropped in Babel v7.

[augustobmoura]

I was targeting 0.12.18 But nervermind, just convinced my team to upgrade the node version, closing this PR