Make all Disposables and Schedulers into classes

Although they can be implemented either way, describing them as classes
makes it clearer that they have identity, and not value semantics.
Remove test for Disposable copying

Now that disposables are objects, copying doesn’t make sense.
Add overrides of scheduleAfter with default leeway

This affords convenience, but with a more power-efficient default than
0.
Use a better default leeway for Signal.interval()

Like Scheduler, we should provide a default leeway of 10% of the timer
interval, for better power efficiency. A leeway can still be manually
provided if this is undesirable.
Dispose of signal observation when await() is done
Add Promise.notify

This is simpler than using Promise.signal.observe, and frees callers
from having to perform silly `nil` checks.
Merge pull request #1489 from ReactiveCocoa/value-types-properly

Make all Disposables and Schedulers into classes
Merge pull request #1493 from ReactiveCocoa/improved-timer-leeway

Use 10% of timer intervals as a default leeway for power efficiency
Merge pull request #1491 from ReactiveCocoa/promise-improvements

Add Promise.notify
Update Quick and swiftz

(Though Nimble is apparently broken.)
Fix raw value conversions for Error enum
Make Optional extensions internal

Required by the compiler.
Upgrade project settings because lol Xcode
Replace swiftz_core with LlamaKit
Use ReactiveCocoa/LlamaKit for important fixes
Re-add our own `identity` function
Migrate to LlamaKit, fuck up all build settings
++xcconfigs for jspahrsummers/xcconfigs#29
++xcconfigs
Merge pull request #1505 from ReactiveCocoa/llamakit

Migrate from swiftz to LlamaKit