Added support for atomic tags; ignore tag attributes; interpret whitespace; pass in wrapper class name

[keanulee]

A few changes that made this library useful for us. Here are the changes:

• Atomic tags - Tags such as <math> and <svg> tags do not support <ins> and <del> tags as children. So during the tokenization step, we consider these tags as a single token.
• Ignore tag attributes - When running the diff, we ignore changes in tag attributes. When two tags are considered equivalent, we output the tag from the after list of tokens.
• Interpret whitespace - Collapse adjacent whitespace (as adjacent whitespace is meaningless in HTML, except for <pre> tags - a case this does NOT handle), and then convert non-breaking whitespace to regular spaces (e.g. ' ' == ' ').
• Wrapper class name - allows an arbitrary class name to be included in the <ins> and <del> tags.
• Tests - To support all these changes.