Added ParsingException superclass and sub classes for parsing state
situations.
Updated UnclosedTagParserException to use standardized form
"UnclosedTagParsingException" instead.
Updated HtmlClosingParser to include different exception thrown
dependencing on opening or closing tag.
Updated HtmlClosingParserTest to include updated functionality.
Updated HtmlParser into abstract class so that it may implement static
methods useful for parsers to know.
Branch Iss #2: Updated HtmlClosingParser. State regarding the HtmlData
is no longer held in the parser itself, but updated via the
MutableHtmlData which is a mutable implementation of HtmlData.