Add Getting Started instructions to the README

Completes #13.
Fixed some style issues.
Added RACTuplePack().
Merge pull request #301 from ReactiveCocoa/setup-instructions

Add Getting Started instructions to the README
Fixed some formatting.
Wrapped ARG in parentheses to force precedence of the expression with respect to :?.
Switched +tupleWithObjects: with +tupleWithObjectsFromArray:.
Merge pull request #305 from ReactiveCocoa/ractuplepack

RACTuplePack()
Merge branch 'master' into better-async-command
Fix unnecessary atomicity
Disallow nil signalBlocks