Implemented some experimental event loops with nextTick() support.

* ExtEventLoop - based on the ext-event PHP extension
* StreamSelectNextTickLoop - based on stream_select
Trying installion of ext-event in travis configuration.
Changed StreamSelectNextTickLoop to use existing Timers implementation.
Removed AbstractNextTickLoop::streamKey()
Abstracted the actual next-tick queue into it's own class.
Reduced method dispatching somewhat.
Favour isset() over array_key_exists().
Removed NextTickLoopInterface, AbstractNextTickLoop and renamed StreamSelectNextTickLoop to StreamSelectLoop.
Various changes as per comments in PR.
Fixed and added test-case for next-tick events added by timers.
Removed some unnecessary left-overs.
Use status for unused parameters.
Added nextTick() support to LibEventLoop (based off ExtEventLoop).
Renamed NextTick namespace to Tick + enable ExtEventLoop tests for Linux.
Added nextTick() support to LibEvLoop.
Use 'inheritdoc' for methods from LoopInterface.
Code style.
Added callable type-hint to LoopInterface, use SplQueue::isEmpty().
Simplification of LibEvLoop.
Added @-operator error suppression to EventBase::loop() calls.