Add command support for UIRefreshControl
Add RACControlCommandExamples to UIKit tests

To be used for UIRefreshControl testing.
Add UIKit tests for UIRefreshControl
Publicize UIRefreshControl+RACCommandSupport.h
Eliminate nested subscription
Verify -endRefreshing is called
Break retain cycle
Revert "Break retain cycle"

Thanks to @erikprice for pointing that I was mistaken.

This reverts commit 113c8b2329071c4fe500e3be838e35b42cd06f1b.
Verify -endRefreshing is called on error
Simplify signal composition for refreshing
Merge pull request #873 from ReactiveCocoa/refresh-control-command

UIRefreshControl command support