Remove escape html

In my rails app, I use sanitization to HTML strings which removes all the harmful tags but lets `<p>` `<li>` etc to pass through. But when I pass the sanitized HTML strings to Diffy for comparison, it escapes all the tags unnecessarily. Retaining those unharmful tags are essential to the layout of the diff view.

Why should Diffy bother about HTML escaping in the first place though! Diff should concern only about strings and delegate the job of HTML escaping to the users. It should be the job of users to sanitize their HTML string before giving input to the `diff.new`. Removing html escaping from Diff will make it lighter, faster and more generic.

This pull request:
1. removes html escaping part of the code, erb dependency and relevant tests.
2. solves "No newline at end of file" problem for `:html_simple`.
3. adds a note on the `README` about HTML escaping.