Basic tests for instantiating/observing a Signal
Add basic test for Signal.interval
Test pipeWithInitialValue
Test optional unwrapping operators
Use curried methods for observation callbacks
Test signal merging and switching
Test mapping
Rename to scanWithStart(), add test
Test take()
Test takeWhile()
Rename to combinePreviousWithStart, test
Test skip()
Test skipWhile()
Test filter()
Test skipRepeats()
Merge remote-tracking branch 'origin/swift-development' into signal-tests
Test and update docs for Signal.buffer
Test Signal.combineLatestWith
(Failing) tests for Signal.sampleOn
Fix memory management in sampleOn test