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

Conflicts:
	modules/swagger-core/src/test/scala/parameter/ParameterSerializationTest.scala
Merge pull request #725 from martoe/params_optional

Fixed the default value for the "required" flag of header-, cookie-, and form-parameters
Java 8 compile fix

Compilation with Java 8 had failed:
reference to Parameter is ambiguous.
both class java.lang.reflect.Parameter in java.lang.reflect
and interface com.wordnik.swagger.models.parameters.Parameter in com.wordnik.swagger.models.parameters match
Failing unittests (Windows) ignored
Implemented vendor-specific fields at Operation and Parameter level
Make response headers available to the code generator
Use an own version number for this fork
Error handling improved