added hyperlinks to Starling's docs, where required (closes #920)
Changed static styleProvider variable to globalStyleProvider to use a different name than the non-static styleProvider property because asdoc hyperlinks don't differentiate between static and non-static names and omitted the non-static property in subclasses of FeathersControl. bonus side effect: haxe users won't complain to me anymore because their compiler doesn't respect actionscript capabilities
StyleNameFunctionTheme: added public dispose function that will clear style providers. subclasses can override to dispose assets.
Header: disposes leftItems, centerItems, and rightItems by default when the Header is disposed to avoid possible memory leak