Adding request and response serializer properties to UIWebView category

Adding documentation to UIWebView category
Documentation fixes

Wrapping iOS 7 / OS X 10.9 only APIs in #ifdef
Adding documentation to UIProgressView category
Adding documentation to UIButton category

Switching backing of UIButton category to AFURLConnectionOperation
Adding documentation for UIActivityIndicatorView category
Updating documentation for UIImageView category
Adding documentation for UIAlertView category
Minor refactoring for AFNetworkActivityIndicatorManager