getBowerConfig was making some bad assumptions about bower.json and main files

-bower.json doesn't always exist, but .bower.json does.
-mainfiles can also include other asset files, like css
-added a warn for when none of the mainfiles are js, or multiple js mainfiles are found.
Merge pull request #9 from billysbilling/better-resolution

getBowerConfig was making some bad assumptions about bower.json and main files