Allow Promises to resolve asynchronously

Stop blocking threads, man.
Rename Observable to Signal

Per discussion in #1382.
Rename Enumerable/Enumerator to Producer/Consumer
Better names for bridging methods
Various naming, asynchrony, and docs improvements
Remove weird synchronous Promise constructor
Nicer Action signatures with ExecutionSignal alias
Bridge RACEvent <> Event
Add NSURLSession extension as proof-of-concept
Add NSNotificationCenter extension as POC
Add modocache/Quick as a submodule
WIP reboot project organization and workspace

Xcode 6 was basically barfing everywhere on the old workspace, so
just start from scratch and do it right.
Get Quick tests sorta kinda working on OS X
Fix up submodule references
Remove stub tests file generated by Xcode
I don't know what's real anymore

OS X wants these parens, iOS doesn’t
Whoops, forgot to commit this workspace
Turns out we don't need a bridging header, oops
++xcconfigs
Add case analysis function for Result type