Deprecated let:
Moved  to deprecated category.
Adjusted deprecation warning for -let:.
Moved clang deprecation suppresion pragmas to implementation file of RACSignal+Operations.
Merge remote-tracking branch 'origin/2.0-development' into pr/555

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa/RACSignal+Operations.h
Merge branch 'pr/555' into 2.0-development (pull request #555)