API to handle "operation already queued" situations

* Added onAlreadyQueued: to MKNetworkOperation to specify
  block(s) to be executed when operation is re-enqueued.
* Defined MKNKAlreadyQueuedBlock which is an argument to
  onAlreadyQueued: and takes already queued operation as
  a parameter
* Documentation updated

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Hi Mugunth,

In my application I need to increase network request's priority if it's re-enqueued.
I couldn't do it easily with current API, so I wrote this small addition.

It allows to specify actions to be executed when engine is trying to enqueue an operation that's already in the queue.
The alternative approach would be to advertise (allow in docs to override it) and maybe rename `updateHandlersFromOperation:` , or just create a separate method that's called from `updateHandlersFromOperation:`. I just went the way the API is constructed and defined handler blocks.

Feel free to merge it if you like, or suggest a better approach.

Cheers!
Dominik