Lossen attribute and reference regex, validate later

[rossj]

This is a potential fix for #12, with all tests passing.

I tried various updates to the Attribute regex that would still include the Reference regex while avoiding the RangeError, but I was unable to avoid either a "hang" (catastrophic backtracking) or the RangeError, so I went with a different approach.

Here's what I've done

• Remove the Reference regex from the Attribute regex, and instead just allow all & characters (references validated later)
• Further loosen the Reference regex to capture any &... with an additional check added in replaceReference

[rgrove]

Thanks!