Basic changes to work with RAC 2.0
First pass, very dirty refactoring

- Start using channels for stuff. Still not totally sure I'm using
  channels correctly, so this will need some looking into soon.

- Totally removed RAFLens.

- Now what was <RAFExtract> is only being used to provide type-safe
  casts from model protocols to their primitive equivalents. So we can
  call it <RAFCast>.

- All table forms must be provided with explicit sections. It's not
  really sane to try to infer these from the form's structure.
Clean up RAFFormlet interface for safer usage

- Remove -channel from the public interface to <RAFFormlet>. Instead,
only expose the terminal that API consumers are expected to use (for
receiving values entered through the UI and for sending values to update
the UI). This resolves the issue of it being really confusing which
terminal to use.

- Move valueTransformer to RAFPrimitiveFormlet, since it has no effect
on RAFCompoundFormlet!
Added -totalDataSignal, analogous to -validationSignal but unvalidated

This merges both terminals of the form's channel (so that you can have
an always-up-to-date signal of the form's data). This is analogous to
the -validationSignal, but it is unvalidated.
-raf_cast is always the identity, so might as well give it for NSObject
Added macro for rapidly mapping primitive models to classes
Get rid of configure blocks from input rows

New contract: if you copy the row, only the stuff about the row will be
kept: the cell needs to be reconfigured. This is actually simpler, and
the fake-prototype-stuff was not as useful as I thought it would be.
Clean up compound formlet channel and validation signal construction
Cruft removal
Begin documenting types of methods
Refactored RAFValidator <: RACCommand; rejiggered constructor

In fact, RAFValidator A <: RACCommand (RAFValidation A).

Also made the by-predicate convenience constructor take a single error.
Removed unused RAFFunctor
Fix signal/channel creation ordering issue

Cleaned up some cruft too.
Update podspec
Expose -[<RAFFormlet> channel], since subclassers will need to override
Fix behavior of -[RAFCompoundFormlet channel]

1. This should send nil rather than NSNull.
2. This should deal with keys rather than indices, since the latter are fragile.
Defer the totalDataSignal for primitive formlets
Make validator a mutable property instead of copying
Tweak signal building code for RAFPrimitiveFormlet
Replay this validation signal