Don't rely on `webdriver.promise` or `webdriver.promise.Scheduler`

[sjelin]

• webdriver.promise.ControlFlow.EventType.IDLE can fallback on "IDLE"
• new webdriver.promise.Promise(...) should fall back on native promises
• isPromise should just be part of maybePromise (not strictly necessary but worth doing since maybePromise needs this anyway)
• initJasmineWd(flow) should have flow as optional and default to a mock flow similar to SimpleScheduler but with only execute() and promise() implemented. This is obviously not the prettiest solution but it's probably best given time constraints.