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

Issue #7 is resolved.

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`.