NSInvocation stuff: no need to use RACSequences
Added real validation! (#12)

Validation means [Success val] or [Failure errors]. Validations may be combined,
in which case errors are accumulated and the leftmost success is kept.

Improved form signal generation; the validation signal is now built in the same
way as the data signal, which is pretty efficient.

Added some algebra protocols (RAFApply, RAFApplicative, RAFSemigroup,
RAFMonoid), which are mostly just used in the validation module.
Documented algebra
Documented some new stuff
Merge pull request #15 from jonsterling/validation

Grownup Validation