Improve `canRemoveAttributeQuotes`

See http://mathiasbynens.be/notes/unquoted-attribute-values for how browsers behave when it comes to unquoted attribute values, and what the HTML spec (based on pre-existing behavior) says about it.

Closes #60 and #61.
Merge pull request #61 from mathiasbynens/gh-pages

More robust check of unquoted attribute values