Updated HtmlParser by removing redundant interface and utilizing
protected methods to ensure that any users using the abstract class as
an interface will not be able to access encapsulated functionality.
Update: Extracted functionality from HtmlParser instances into abstract
class
Updated HtmlAttribute to include toString method, equality method, and
methods for setting up special closing tag, and the default values and
constants associated.
Updated parsers.errors superclass ParsingException to operate correctly.
Updated MissingTagParsingException to MissingEnclosureParsingException
to allow catching of quote enclosures as well.
Updated constant locations from AbstractHtmlParser to HtmlParser.
Removed unnecessary clutter classes.
Branch Iss #2: Step eight. Provided HtmlAttributeParser and its
associated testing class.