v2.0.1: Scrolling logic tweaks

• Improved code reuse in the logic which tries to determine whether or not a MarqueeLabel instance should scroll. This should also make debugging easier, as you can set breakpoints on specific lines to determine why your label isn't moving.
• Added overridden willMoveToWindow: and didMoveToWindow UIView methods. These methods are used to hopefully improve stopping scrolling animations when they're no longer needed (which helps prevent jumpiness if they're re-added to a window) and to better restart scrolling during view controller transitions.
• Re-implemented the modal view controller in the demo.
• Added label pause switch to demo.
• Corrected some other errors in the demo.