Add Specs
Replace - [RFField validate] : RACSignal method with - [RFField validate:] : BOOL method
Create category on NSOrderedSet with method for set diff
Remove RFDefines.h from precompiled header
Add RFCollectionOperations category simplifying Cocoa collections transformations
Remove Core Data backing structure
Add mapWithIndex and foldLeft operations to RFCollectionOpeartions
Factor common functions in RFForm changes signal
Improve - [RFSection description] and - [RFField description]
Encapsulate NSOrderedSet  diff result behind protocol
Rename methdo - (RACSignal *)visibleElements to - (RACSignal *)visibleFields.

This names better reflects what values signal sends
Add - [NSObject do:]
Add RFFormObserver protocol. Add interface for observers management to RFForm

This commit will probably require deprecation of RFFormController.
Fix failing tests
Change RFFormElement protocol to require NSArray of fields instead of RACSequence

This is premature step due to deprecation of RACSequence support in
future releases of ReactiveCocoa
Update RFSectionSpec
Create draft of RFFormSpec