Enhancement (DELETE) and Bug fixes (around POST response)

1. In fetch(), the check for negative response to raise HALNavigatorError,
       if raise_exc and not self.response:
            raise HALNavigatorError(self.response.text,
                                    status=self.status,
                                    nav=self,
                                    response=self.response,
            )    
   Is not too late in the flow?. I feel it should be checked before processing the response body 

2. PostResponse can have properties, hence implemented __call__ to return properties

3. DELETE method is similar to POST from Navigator perspective (non-idempotent) 
  Hence,
         a) Reused logic from POST to implement support for DELETE
         b) renamed PostResponse to HALResponse 
         c) renamed  create() to post() for clarity
   In order to provide backward compatibility, I created alias "create"  to "post"

4. HALNavigator should not allow POST if the uri is templated. The check for templated_uri 
   was missing. Created a wrapper function (@template_uri_check) to re-use for all methods - GET/POST/DELETE

TODO:
1. Enforce templated_uri check for all HTTP calls. Add test coverage 
2. a link usually have 'method' property (default to 'GET'), which can be used to restrict http operations. this helps to raise NotImplementedError and can have a controlled default function e.g autofetch
again, I don't think we should bake a method key into links, since it isn't part of the HAL spec