Add foldr implementation

Add a lazy foldr implementation and tests for it. The lazy foldr uses an
RASequence to wrap the lazy computation while making it easy to
retrieve.
Add any function

Add any function using the previously implemented foldr.
Add tests for the any function.
Refactor fold interface
Add RACProxy

RACProxy provides an unobstrusive wrapper over an object that should be
created lazily. The object inside the proxy is lazily generated from the
block on the first selector and all messages are forwarded to it.
Revert "Add RACProxy"

This reverts commit 669c3bcc6764e76bfc913554e008bf8fe7e2d488.
Rename foldRightWithStart to foldRightLazilyWithStart
Add foldRightWithStart (non lazy)
Add foldLeftWithStart:combine:
Remove non-lazy fold
Add non lazy foldr specialization for EagerSequence
Migrate any to use foldLeft
Rename foldLeft param for clarity
Merge branch 'fold' of https://github.com/aitskovi/ReactiveCocoa into aitskovi-fold
document the parameters
asserts + style
style
one more style
doc the block parameter
style yo
added -execute: