Create CallHandler annotation for Proxy methods

[lidaamber]

We should add a @CallHandler annotation for Proxy methods to process incoming calls in the way like this:

@CallHandler
void onMethodCalled(List<?> args);

@CallHandler("methodName")
void onMethodCalled(List<?> args);

@CallHandler({"interfaceName", "methodName"})
void onMethodCalled(List<?> args);

With this annotation the generated class for Proxy should be abstract, so that the User'll later implement handlers for methods with @CallHandler annotation.

[lundibundi]

see #19 (comment).