Fixed single quotes issues, added tests, removed unused using and code cleanup

Besides the minor changes, expect zencoding to be more flexible when having double quotes, single quotes and mixture of them in attributes and values.

For an instance

    p[title="This test has double "quotes""]

will output

    <p title="This test has double &quot;quotes&quot;"></p>

Similarly the apostrophe S in single quoted string would be accepted and transformed to `&#39;s`.