Add <~ binding operator for HotSignal
Add <~! binding operator for ColdSignal
Upgrade startMulticasted error handler to be fatal
Test property binding operators
Change ObservableProperty.values into a property

It doesn’t have side effects upon invocation, so there’s no real reason
for it to be a method.
Merge remote-tracking branch 'origin/swift-development' into property-binding-syntax

Conflicts:
	ReactiveCocoa/Swift/HotSignal.swift
Merge pull request #1600 from ReactiveCocoa/property-binding-syntax

Add <~ and <~! operators for binding properties, change `values` into a property