runtime: Add RecurisveRewatcher trait and stub for it
Refactor Subscriber -> WatchPoint

WatchPoint is easier to spell than Subcsriber and auto-completion have
worsen in vim recently.
runtime: Refactor ctor + test fixture
Add WatchPointTree - WalkPoint method
Check for dangling marks in TestWatchPointTreeWalkPoint test
Add resumable WalkPoint with CWD
Add Recursive internal event to stubs
Reconfigure Travis CI to build under Mac OS X