Extend RACAble underlaying methods by two additional arguments

These two arguments `changeKind` and `option` will allow better observation
of key-value compliant properties, including observation prior change, obtaining
previous value of the property or observation of collection mutations.
Add RACTuplePackWithNils that converts NSNulls to nils
Add variety of RACAble macros with backward compatibility

New RACAble macros for returning previous value (not only current one),
observing prior setting (the old value) and observing collection changes.
Underlaying macros and methods now accept new argument that defines when and
what objects are send from this signal.
Make KVO signal creating with types more readable

New method `+rac_signalWithChangesFor:keyPath:observer:` returns a signal, that
returns whole change dictionary from KVO. Modified method
`+rac_signalFor:keyPath:observer:type:` appends `-filter:` and `-map:` streams
depending on the given type.
Rename RACAbleTypeReplacement -> RACAbleTypeReplace
Merge branch 'feature/kvo-options' into develop
Fix detection of initial KVO observation
added -rac_signalForSelector:
don't need that anymore
clarify wording
more clarification
assert if the method has an existing implemenation
send the argument instead of the receiver
corrected the method signature
I guess conditionalize the test based on asserts
mark unused
derp
Add convenience method for delayed schedule on RACScheduler

Method `-after:schedule:` takes dispatch_time_t argument that is in nanoseconds.
New method `-delay:schedule:` takes NSTimeInterval as argument (in seconds) and
calls `-after:schedule:` with calculated dispatch time. Method is implemented
in suberclass and is not abstract.
Update README.md
implemented forwarding of completed and error events to RACGroupedSignal when using -groupBy: