Http DELETE support, lightweight callbacks.

[akolechkin]

Hello.

I would like to suggest some changes in rocket.

1. rocket/http_handling.py
   When i worked with sailthru.com i found that rocket not working with send.delete.
   I've investigated that problem, and drop on a problem with urllib2. It doesn't support http DELETE method out of box. I created a successor of urllib2.Request class, with various http-methods support.
   Another issue, i found, you can see here https://github.com/exfm/rocket/blob/master/rocket/http_handling.py#L81
   RocketAPIError is not in namespace, and i couldn't find implementation of this class, as well i don't know behavior of this class.

2. rocket/rocket.py
   It looks like an improvement, for easy support of lightweight callbacks to a method.
   f.e I would like to handle any error that could happen at remote api call.
   I can write a handler like that:

   def my_error_handler(event):
       """
       event - is rocket.Event class instance.
       """
   
       if event.succeed:
           # means no exception here
           write_in_db(event.response.somevalue)
       else:
           # means exception here
           logging.critical('Error at sending %s %s' % (event.method, event.ns_fun)
   
   api = Sailthru(...)
   api.add_callback('post', 'send', my_error_handler)        
   
   #otherwise, if i want to handle all the calls:
   
   api.add_callback(None, None, my_error_handler)        
   
   #or any post callback
   
   api.add_callback('post', None, my_error_handler)