Upgrade to google-cdn v0.2

I spent some hours this weekend adding support for different CDNs to `google-cdn` (which should definitely be renamed). I also got rid of the hard-coded main files and resolve them through bower, which required an API change to an asynchronous interface.

Currently only CDNJS is supported, because JSdelivr does not expose any but the most recent version and doesn't have *main* file. I guess I could work around both those limitations, but wanted to get this out as a first step.

/cc @btford
/fixes #30