time: deal a bit better with time zones in Parse

* Document Parse's zone interpretation.
* Add ParseInLocation (API change).
* Recognize "wrong" time zone names, like daylight savings time in winter.
* Disambiguate time zone names using offset (like winter EST vs summer EST in Sydney).

The final two are backwards-incompatible changes, but I believe
they are both buggy behavior in the Go 1.0 versions; the old results
were more wrong than the new ones.

Fixes issue 3604.
Fixes issue 3653.
Fixes issue 4001.

R=adg
CC=golang-dev
https://codereview.appspot.com/7288052