Handle broken addresses in ToHostName filter more gracefully

We had a problem manifesting in this traceback:

Traceback (most recent call last):
  File "bin/postoffice", line 111, in <module>
    repoze.postoffice.script.main()
  File "/srv/multikarl/production/15/eggs/repoze.postoffice-0.21-py2.6.egg/repoze/postoffice/script.py", line 75, in main
    return ConsoleScript()()
  File "/srv/multikarl/production/15/eggs/repoze.postoffice-0.21-py2.6.egg/repoze/postoffice/script.py", line 48, in __call__
    po.import_messages(self.log)
  File "/srv/multikarl/production/15/eggs/repoze.postoffice-0.21-py2.6.egg/repoze/postoffice/api.py", line 177, in import_messages
    self._import_message(message, log)
  File "/srv/multikarl/production/15/eggs/repoze.postoffice-0.21-py2.6.egg/repoze/postoffice/api.py", line 220, in _import_message
    if not filters or not _filters_match(filters, message):
  File "/srv/multikarl/production/15/eggs/repoze.postoffice-0.21-py2.6.egg/repoze/postoffice/api.py", line 396, in _filters_match
    if not filter_(message):
  File "/srv/multikarl/production/15/eggs/repoze.postoffice-0.21-py2.6.egg/repoze/postoffice/filters.py", line 28, in __call__
    addr = addr[lt+1:addr.rindex('>')]
ValueError: substring not found

This stopped any incoming mail processing.

The attached patch fixes it by accepting anything after the "<" as the potential email address and avoids getting stuck.