Change RAC_DEBUG_SIGNAL_NAMES into an envvar
Vary -rac_description behavior based on the envvar too
Use sel_getName in signal naming to avoid allocations

Selector names are cheaper to retrieve as C strings than to wrap in
NSStrings.
Update instrumentation README
Merge pull request #1114 from ReactiveCocoa/signal-naming-envvar

Change RAC_DEBUG_SIGNAL_NAMES into an environment variable