added api-key
 [2.0 Spec] Implement Schema Object.example field #699
Unittests fixed

* ModelConverterTest, ScalaModelTest: the properties ordering is different (alphabetic as it seems)
* ParameterSerializationTest: comparison failed because of CR/LF (could also be an OS-related problem)
* CompositionTest did never work in history (maybe an implementation code is missing?)
Header, cookie, and form parameters should be optional

According to the spec
(https://github.com/wordnik/swagger-spec/blob/master/versions/2.0.md#parameterObject),
only "path" parameters are required by default, while for
all other parameter types the default value is "false".
Header, cookie, and form parameters should be optional

Unittests fixed
Implemented vendor-specific fields at Operation and Parameter level
Make response headers available to the code generator (tryout)