Update API docs and demos for recent worker improvements.

>Not unlikely at all, especially since you assumed that users will forget to call .close()

I'm not assuming they will forget *all* calls to .close(), but they are likely forgetting some of the calls, especially in error handling codes. And it is likely that some codes before .close() may throw.

>One too many.
Still better than refcounting, because even if a single .close() call is missed for any reason, the worker is doomed to persist with the page.

>Resource leaks.
A timer consums very little resource. JS code of zip.js itself would consume resources, and it persists with the page.

>And defeats the purpose of pooling, namely controlling/constraining the number of active workers in use by zip.js

It does constrain the number of active workers to <= 1.