Avoid using copying property as a block value

This may fix #505 leak issue.
Restore the type signature for dependencyBlock
Explain why the block properties are "strong"
Merge branch 'pr/506' (pull request #506)
Merge branch 'master' into ios-testing-target
Revert "Extracted the NSOperationQueue RACBacktrace test."

This reverts commit 708341f140352c648bb3c1e178e4dc342afa8702.
Wrapped NSOperationQueue test in platform conditional.
Reverted refactoring of RACLifting specs.