Implement -[UITableViewHeaderFooterView rac_prepareForReuseSignal]
Extract TestTableViewController
Add test for UITableViewHeaderFooterView+RACSignalSupport
Use dot notation
NSStringFromClass() instead of +description
Remove unnecessary property/implementation
Merge pull request #1040 from ikesyo/uitableviewheaderfooterview

Add UITableViewHeaderFooterView support
Allow nil inner signals when flattening
Merge pull request #1045 from ReactiveCocoa/skip-nil-flattened-streams

Allow nil inner streams when flattening