Queueing system for generic requests

[davefinster]

Much like the RKImageLoader, it may be preferable to queue multiple RKRequest objects so that only one is executing at a time. Preferably the RKRequestManager should control this and also have the ability to enlarge the number of simultaneous requests being processed. Once this has been implemented, transition the RKImageLoader queueing system over to RKRequestManager so that it's only task is to handle callbacks.