Google Earth Imagery Provider

This adds support for using Google Earth as an imagery provider.

I wasn't sure if the class should be called `GoogleEarthImageryProvider` or `GoogleEarthEnterpriseImageryProvider`. Google has Google Earth Enterprise and Google Earth Pro products that they provide, but it is unclear if this imagery provider will work with the pro version (it has only been tested with the enterprise version).

I wasn't able to get the test for `raises error event when image cannot be loaded` working correctly. It's been a while since I looked at that problem, but if I remember correctly, it was because I wasn't able to use the jsonp metadata callback because the Google Earth server returns malformed JSON. I could use some help getting the Jasmine spec to work with that case.