Adds build script to create and publish the bower repo

Hi, we at platanus have been running the `platanus/bower-mapboxjs` repo that is being used by the `mapboxjs` package from bower. Sadly we're not using mapbox to actively so the repo usually gets outdated.

I've build a script that automate the bower building and publishing trying to solve this issues platanus/bower-mapboxjs#2 and #404 and #740

If you are willing to merge the PR you can fork our repo, or if you prefer I can transfer the ownership to you.

Also we can change the endpoint to `mapboxjs` in bower

I hope this is useful for everyone