Allow dots in trigger instance payload (bug fix)

[Kami]

Previously, rules engine would explode if a TriggerInstance dictionated payload contained a dot or $ in the key name.

640fd7215ce2f1a', u'end_timestamp': u'2015-05-11T21:33:10.971247Z'}}, 'execution_id': '555124210640fd7d31ab0409'}}
Traceback (most recent call last):
  File "/data/stanley/st2common/st2common/transport/consumers.py", line 59, in _process_message
    self._handler.process(body)
  File "/data/stanley/st2reactor/st2reactor/rules/worker.py", line 32, in process
    datetime.datetime.utcnow())
  File "/data/stanley/st2reactor/st2reactor/container/utils.py", line 62, in create_trigger_instance
    return TriggerInstance.add_or_update(trigger_instance)
  File "/data/stanley/st2common/st2common/persistence/base.py", line 83, in add_or_update
    model_object = cls._get_impl().add_or_update(model_object)
  File "/data/stanley/st2common/st2common/models/db/__init__.py", line 125, in add_or_update
    instance.save()
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/mongoengine/document.py", line 224, in save
    self.validate(clean=clean)
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/mongoengine/base/document.py", line 323, in validate
    raise ValidationError(message, errors=errors)
ValidationError: ValidationError (TriggerInstanceDB:None) (Invalid dictionary key name - keys may not contain "." or "$" characters: ['payload'])

This also resolves #1368