Unit test for format preserving

Test that Premailer outputs the same format as it's given, e.g.
ElementTree input will give ElementTree output.
Make Premailer output the same type that it was given.

Works with either strings or ElementTree objects.

This can save time re-parsing the document if other operations need to
take place before or after Premailer.
Merge pull request #96 from moggers87/etree-in-etree-out

Accept ElementTree objects