Added diff output to xml_equal

I was unsure of my approach to diff generation but, unless something more sophisticated is needed for the other methods, I think it's fine. These commits generate better formatted diff output. Thoughts @apotonick?

It might not be a bad idea to make `:context` configurable...

I'll await your feedback before merging. 

Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.