improvements to Android XML generation

There are a few improvements two the generation of the Android resources XML here. A few are functional:
 * the XML color declarations should be wrapped in an `<resources/>` tag
 * fully opaque colors should have an alpha of `0xff`, not `0x64` (100 decimal)
 * color names should_contain_underscores, not camelCasing

Also, because Ruby, I changed the XML generation to use [Builder::XmlMarkup][xb] instead of string interpolation.

And, finally, I removed the `cocoapods` dependency from `Gemfile`. `cocoapods` has a transitive dependency on something called `xcodeproj`, which can't be installed on Linux or other non-OSX computers. And anyway we don't currently seem to use the gem anywhere except in the Gemfile. If this changes in the future, we can consider investigating solutions.
[xb]: https://github.com/jimweirich/builder