Replace DustJS

[ianfixes]

DustJS seemed great at the time we wrote this whole thing, but we don't really love it anymore. There's probably a better templating language for a case like this where whitespace matters (for example) - so you should suggest it to us.

[GriffinSchneider]

swagger-codegen does code generation using mustache templates and produces reasonably-formatted code, so that's probably worth looking at:

https://github.com/swagger-api/swagger-codegen/tree/master/modules/swagger-codegen/src/main/resources/objc