Rewrite -concat to avoid a recursive block
Add debug logging methods to RACSignal
Merge pull request #237 from github/concat-block-cycle

Rewrite -concat to avoid a recursive block
Merge pull request #240 from github/signal-logging

Add debug logging methods to RACSignal
Merge branch 'master' of https://github.com/github/ReactiveCocoa
Style errors corrected.