Properly parse bower.json

Some work is needed to parse the `bower.json` into a format acceptable by jspm so that all dependencies are retrieved. [Example bower.json](https://github.com/twbs/bootstrap/blob/master/bower.json). Open questions:

1. What can we do about packages with multiple `main`s?

2. Is it ok to use the same semver logic as the npm endpoint?


 cc @guybedford