Added method to get length of StrokeEx

Adding the ability to measure the length of the StrokeEx line created.
This value can be used for, for example, deciding how to stretch a
material along the curve, allowing for increased flexibility for effects
along a curve.
Update to fix betBounds bug

Fix for Issue #50

https://github.com/StarlingGraphics/Starling-Extension-Graphics/issues/50

As soon as we add a vertex, none of maxBounds and minBounds can be
invalid ( and yes, INFINITY is invalid in this scenario ;) With only one
vertex, maxBounds and minBounds should be identical, That's how a
mathematical point works ;)
Merge pull request #57 from IonSwitz/master

Fix for getBounds bug, Issue 50