WIP Hook for NSURLSession
Ensure hook swizzles concrete subclass of NSURLSessionConfiguration.

This commit swizzles the actual instance of NSURLSessionConfiguration, which is
__NSCFURLSessionConfiguration. Tests now pass.

Signed-off-by: Luis Solano Bonet <luis@pixable.com>
Use exchangeImplementation for swizzling
Clenup tests
Update README
Include the ASIHTTPRequest hook out of the box.
Merge pull request #60 from luisobo/nsurlsession-hook

[WIP] Hook for NSURLSession