Making npm package name lower case and adding a peerDependency on mutationobservers

This package is now published to npm I will remove it when the Polymer team publish the packages, I had to start publishing the packages because peerDependencies cannot be simple GitHub URLs as they need to have version information declared as the value of the package key.

`"mutationobservers": ">=0.0.0"`

I couldn't have a GitHub URL where the version is as that prevented me from specifying a version...

https://npmjs.org/package/customelements