Adding bootstrap.js to default Phoenix installation

This note applies to Phoenix 1.2.0

mix phoenix.new starts you out with Twitter's bootstrap. However, it only includes the CSS, not the JavaScript. I had a good deal of trouble figuring out how to get that working, not being familiar with the JavaScript ecosystem or the brunch build tool that Phoenix uses.

Thanks to the kind help of OvermindDL1, I got it working. Here are the changes, shown in the github diffs.