Move widget time options into a new directive.
Make global->widget time propagation consistent.

The global time range was already propagated to individual widgets via broadcast,
while the endTime was passed as a directive argument. I changed the latter
to broadcast as well.
Allow configuring Graphite graph times via PromDash.
Merge pull request #110 from prometheus/feature/graphite-time

Move per-widget time settings to directive and support setting Graphite time