added intent validator, intent post processor, context handler and de…

[manojkumar1412]

…faultEars

1. Context Handler - A context handler interface to get the context object and send it to wit,
   It has a function with below signature
   function getContext(bot, message); - return context object
2. defaultEars - If wit fails to give any intent then this may help to do some processing over wit,
   may be a regex based botkit defaultEars can be used to do regex matching if wit fails
3. validateIntent - a Validate Intent interface to validate the intent from wit,
   it requires to do some validation over it.
   function validateIntent(entities) - return true if valid else false
4. postProcessIntent - a post process intent interface to do some post processing over wit intent
   function postProcessIntent(entities) - return entities after post processing