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:`
Remove inclusion of config.h

Other libraries define the HAVE_CONFIG_H condition to include
config.h, such as LibYAML or YAML.framework. When this happens
ReactiveCocoa’s extobjc tries to include its own config.h file which
doesn't
exist. As this doesn’t seem to be used in ReactiveCocoa, we can safely
remove it.
Merge pull request #1061 from Breek/remove_config_h

Remove inclusion of config.h
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.