APIs and Models both have classVarName as well as instanceVarName

Slight modification from https://github.com/swagger-api/swagger-codegen/pull/323 (I can't seem to re-open that pull request).

I created templates for a Java client built by Gradle, with fault tolerance from Hystrix and templated by Feign. See: https://github.com/swps/swagger-codegen-templates

I need APIs to have `classVarName` and `instanceVarName` in their mustache template context (like models do), and in addition, give models `instanceVarName` to be in alignment with APIs.

Also, allow for processing scala server pages (.ssp) files for more complicated templating that mustache can't provide.