Changes for #358

* Rewrite ‘tasks’ example as ‘threads’
* Remove mention of green threads
* Fix example to sleep after spawning threads to display prints in
browser
* Rewrite ‘channels’ to use ‘thread’ in text and code
Merge remote-tracking branch 'upstream/master' into task-rework
threads: join threads instead of sleeping
Merge pull request #359 from vbuslov/task-rework

Changes for #358