Add unicode support by replacing str() with unicode() and using utf-8.

str() will not accept non ASCII characters, such as Δ.
"encoding='utf-8" argument passed to tostring() ensure XML is encoded as utf-8, as per Clockwork SMS API.
Does not enforce character set (http://www.clockworksms.com/doc/reference/faqs/gsm-character-set/).
Add GSM characters test case for a unicode message.

Form feed and escape break lxml, so they are not supported.
Encode balance request message as utf-8 for consistency.