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:`
Fixing RACDynamicSignal.h import
Clarify comments around dynamically implemented `-methodSignatureForSelector:`
Move `objc_msgSendSuper` cast to it’s own line
Merge pull request #1066 from alexfish/fix-dynamic-signal-header

Fixing RACDynamicSignal.h import
Fixed some things.
Remove RACCommand.observationInfo override

Although storing this information on the object directly is supposed to
improve performance, it seems to introduce EXC_BAD_ACCESS crashes in
some KVO methods.

Whether it’s the direct cause or just exposes some underlying issue is
unclear, but we can at least reduce the incidence by going through the
global KVO machinery.
Merge pull request #1070 from ReactiveCocoa/normal-command-kvo

Remove RACCommand.observationInfo override
Merge pull request #1058 from ReactiveCocoa/signal-selector-class

Signal selector class
Remove the disposable from self and the observer when disposed of.
Re-use the same dealloc signal.
Revert "Re-use the same dealloc signal."

This reverts commit 16a81f328955222e88f0ed458fcb741e6f95aea7.
Re-use the same dealloc signal.
Don't need to weakify/strongify.
Just use _cmd.
Merge pull request #1072 from ReactiveCocoa/remove-disposables

Remove the disposable from self and the observer when KVO observation is disposed of
Merge pull request #1073 from ReactiveCocoa/reuse-dealloc-signal

Re-use the same dealloc signal.