TrampolineScheduler & Unsubscribe

Unsubscribing should prevent new additions to a Worker, but not prevent already scheduled work, and definitely not affect other Workers using the same thread (by modifying the ThreadLocal as it was doing).

See the unit test for details of how unsubscribing 1 Worker could prevent work from being done on a completely separate Worker.
Merge pull request #1324 from benjchristensen/trampoline-schedule-unsubscribe

TrampolineScheduler & Unsubscribe
Javadoc changes:
* flesh out and update @throws sections
* add @warn nags to missing javadocs
* standardize formatting
* fix dead links to images
Filling in javadoc gaps where I feel confident enough to do so...