Added a tag property to AFURLConnectionOperation

Added a 'tag' property (NSInteger) to AFURLConnectionOperation to easily track different requests.

In our scenario, we have multiple AFHTTPRequestOperation's launched at the same time, and all of them have the same callbacks. We need a way to distinguish them easily, in order to process the output data. Adding a 'tag' property seems like an easy and efficient way to do that.

(A similar property was also found in the old ASIHTTPRequest)