Email Template signature

[Paulius Sladkevičius @ hbee]

If "Use Signature" in email template is checked and user without signature send email, he/she get error.

bin/addons/poweremail/poweremail_template.py", line 945, in _generate_mailbox_item_from_template
[2010-11-17 01:17:58,788][course3] ERROR:web-services:[55]: mailbox_values['pem_body_text'] += sign
[2010-11-17 01:17:58,788][course3] ERROR:web-services:[56]: TypeError: coercing to Unicode: need string or buffer, bool found

I hope this simple patch fix this problem.