runtime: Remove wip tests
runtime: Watching/unwatching directory path
test: Subscribe channels to events in Runtime fixture explicitly
runtime: Add Subscribers + refactoring

- rewrite (*Runtime).Watch to use Subsribers
- rewrite (*Runtime).cachepath to slice instead of map to have
  deterministic path iteration order during (*Runtime).Stop (mainly
  tests)
runtime: Remove cache for total event set per path
test: Rework test.ExpectCalls to execute test-cases for all Runtime implementations
Rename Fanin -> Dispatch
runtime: Dispatching events back to user channels
Merge pull request #39 from rjeczalik/runtime

First working, non-recursive runtime (part 1)