Add CodegenProperty.nameInSnakeCase

Created by: jmini

Raised in #22, it would be nice to have CodegenProperty.nameInSnakeCasesimilar to CodegenProperty.nameInCamelCase.

Example:

baseName	nameInSnakeCase
petId	PET_ID
name	NAME
some_value	SOME_VALUE

When this feature is implemented, #21 (closed) can be changed to be conform to the java naming convention.