Merge lp://qastaging/~halls/poweremail/no-sig-fix into lp://qastaging/poweremail/trunk

Hi, I set up poweremail as I said to you I would do after your great presentation at the conference :)
I tried it with a user without a signature set and got an exception:

ERROR:web-services:[16]: Traceback (most recent call last):
[...]
ERROR:web-services:[22]: return getattr(object, method)(cr, uid, *args, **kw)
ERROR:web-services:[23]: File "/usr/lib/openerp-server/addons/poweremail/poweremail_send_wizard.py", line 161, in send_mail
ERROR:web-services:[24]: mailid = self.save_to_mailbox(cr, uid, ids, context)
ERROR:web-services:[25]: File "/usr/lib/openerp-server/addons/poweremail/poweremail_send_wizard.py", line 211, in save_to_mailbox
ERROR:web-services:[26]: vals['pem_body_text'] = tools.ustr(vals['pem_body_text'] or '') + signature
ERROR:web-services:[27]: TypeError: coercing to Unicode: need string or buffer, bool found

This fixes that by only appending the signature if one is set.