Initial hack at enabling future triggers.

Addresses #487; example in examples/future-trigger runs.
Removed misleading test suite title.
Tidied one code snippet.
Tidied another code snippet.
Merge branch 'master' into 487.future-task-triggers
Merge branch 'BugFix.HoursOfTheDay-cycling' into 487.future-task-triggers

Conflicts:
	lib/cylc/cycling/HoursOfTheDay.py
A better future trigger example suite.
Tidied offset code snippets further.
Handle future triggers in conditional expressions.
Allow shutdown with future-triggered tasks held before stop time.
Merge branch 'master' into 487.future-task-triggers
Merge branch 'master' into 487.future-task-triggers
Added sim mode test. Fixed self.run_mode. Corrected suite spec.
Corrected config.py to pick up overridden run types.
Tweak scheduler code.
Stop command option to kill active tasks before shutdown.
Stop --kill option for gcylc.
Made polling configurable by task, with interval lists.
Completed documentation of polling.
Reordered some runtime items in the suite def reference.