doc: consolidate timers docs in timers.markdown

doc: consolidate timers docs in timers.markdown

Rather than attempting to keep two versions of docs for timers up to
date, keep them in timers.markdown, and leave references to them in
globals.markdown.

Add setImmediate and clearImmediate to globals.markdown.

Change "To schedule" to "Schedules" in timers.markdown.

PR-URL: https://github.com/nodejs/node/pull/5837
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>