Hide the fact that we've been dynamically subclassed.

h/t @keithduncan
Test for the class reporting.
Swizzle -methodSignatureForSelector: to claim we implement anything.
Implement a generic version of `-methodSignatureForSelector:`
Clarify comments around dynamically implemented `-methodSignatureForSelector:`
Move `objc_msgSendSuper` cast to it’s own line
Fixed some things.
Merge pull request #1058 from ReactiveCocoa/signal-selector-class

Signal selector class