Code generator extension mechanism

Added a mechanism that allows developers to add extensions to the code generator without modifying it.  It works by setting the property `<generatorExtensionClass>` in the maven plugin to the fully qualified name of a class that implements `org.raml.jaxrs.codegen.core.ext.GeneratorExtension`.  
* The method `onCreateResourceInterface` is called immediately after generating the Java interface for a resource
* Similarly, `onAddResourceMethod` is called after generating the Java method that corresponds to a resource action.  
* Lastly, `AddParameterFilter` is called when a new Parameter (e.g., URLParameter, QueryParameter, etc). is created.  The extension can filter out a parameter by returning false to this call.

I have not integrated the extension mechanism with the command line or eclipse invocation points but can do so if you will incorporate.