Fix `multiway`'s behavior when dealing with zero-valued axis of `speed`.

This fixes the behavior of `multiway` when `speed` is as an object
with 0 value for either axis. Passing `{x: 0, y: 0}`, or variations
of, is now safe.
Merge pull request #521 from ericathegreat/develop

Fixing reference to removed "Drawmanager.draw()" function

draw() replaced by trigger("RenderScene")
Making other tests run as part of the Grunt task.
Fixing a bug in centerOn for non-zero origins
When the viewport is not already at zero and .centerOn
is called, it was not correctly centering. The further
away from the origin the viewport was, the further away
from the center target it would end up.
Merge pull request #523 from ericathegreat/non-running-tests

Making other tests run as part of the Grunt task.
Merge pull request #522 from ericathegreat/centerOn-scrolling-bugfix

Fixing a bug in centerOn for non-zero origins
Merge pull request #516 from mnemotic/speed-fix

Fix `multiway`'s behavior when dealing with zero-valued axis of `speed`.