Xcode 6 support, various enhancements and fixes

Before The Great Swiftening (ReactiveCocoa#1382) began.

See ReactiveCocoa#1369 for discussion.

Merge pull request ReactiveCocoa#1251 from TimurBK/master

Added RAC support for UIImagePickerController.

Observe id<Protocol> deallocation, disposal and memory improvements

Guard with RAC_DEBUG_SIGNAL_NAMES, fire -rac_textSignal when clearing

Fix -methodSignatureForSelector: for dynamically subclassed classes

Reduce memory usage in rac_willDeallocSignal and KVO subscription

Fix -class in -rac_signalForSelector:, RACCommand KVO crashes

NSUserDefaults and UITableViewHeaderFooterView support, many fixes

Buncha bug fixes