various updates

addMarker now requests layout pass to update marker map rects (to fix
runtime addMarker bug)
resetDetailLevel now updates tile set to prevent false matches
added .pause() sugar method (mirrors .clear())
added .refresh() method (reevaluates tile set, tiles, samples, etc)
updated readme
fixed positioning and scaling issues

positioning methods need explicit scaling;
runtime scaling (not due to gestures) could cause View to scroll beyond
visible limits - added constraint in layout pass to prevent this
updated readme
updated refresh method

added invalidate and clip enforcement to TileView.refresh method
updating slideTo... methods

some devices require invalidate to be called immediately following a
scroller.startScroll *and* in computeScroll.