v2.2.0: Manual scrolling and background color fixes

• Adds a triggerScrollStart method which allows override any non-size condition preventing the label from scrolling when also oversized. Currently, the tapToScroll and holdScrolling properties are the only non-size conditions which prevent scrolling. Note: This doesn't let you force a short string to scroll! (Thanks @pip8786!)
• Corrected an apparent issue which would prevent label background colors from working when set via Interface Builder.
• MarqueeLabel now sets it's own background color in addition to the scrolling sublabel when appropriate (i.e. when the background color is not set to clearColor). This should prevent unnecessary non-opaque views, which can be a performance hit especially in table views. Thanks Mike C.!
• Adds IBInspectable to properties with supported types, so you can set them directly in Interface Builder! (Thanks @yoiang!)