Update for backpressure

[davidmoten]

Code needs review and update to handle introduction of backpressure in RxJava that happened ages ago

[davidmoten]

Also want to make thread blocking configurable. At the moment there is a .take() call on the WatchService which blocks the thread indefinitely (till an event occurs). Would be nicer to use poll(time, unit) for a configurable small time period and then trampoline schedule a new one. This would avoid the request blocking that can occur when a thread-hogging source is operating upstream of a subscribeOn operator.

[davidmoten]

fixes committed for WatchEvents from a WatchService and released to Maven Central in 0.4.1.

Still need to bring file tailer up to date also.