removeFormat: Rename |styleElement| to |styledElement|, since it isn't STYLE element.
removeFormat: Transform elements which have STYLE attribute to corresponding element, e.g. <span style="font-weigth: bold"> to <b>.
Fix #25
removeFormat: Rename |effectiveNodes| to |targetNodes|.
Introduce |EditingContext.prototype.removeNodePreservingChildren(node)|.
Introduce |COmmandContext.hasNoAttributeOrOnlyStyleAttribute(element)| and |CommandContext.isSpanWithoutAttributesOrUnstyledStyleSpan(node)|.
removeFormat: Get rid of useless SPAN elements at selection start.
Fix #24