Remove account requirements

[minsis]

Fixes #45

[CLAassistant]

All committers have signed the CLA.

[amanda11]

Is the change to config.schema.yaml is enough? I'd have expected when the schema performed a
self._parse_accounts(self._config['imap_accounts'])
then it would get back None.

Therefore when it then tried to iterate through None it would get an uniniterable type.

So I would think the two sensor files would need to change to in the config, set it to [] if its not set.

Then the pack.yaml and CHANGES.md also needs an update.

[minsis]

Maybe? I thought a poller would only start if there was an associated rule trying to use it. So if you never created a rule to use it, the poller wouldn't start.

Also, there's already logic surrounding it:

stackstorm-email/sensors/imap_sensor.py

Line 48 in ed66fd0

if 'imap_accounts' in self._config:

[nzlosh]

imap_accounts might be able to be made optional but smtp_accounts is a required part of the send_email code.

stackstorm-email/actions/send_email.py

Lines 18 to 22 in ed66fd0

	accounts = self.config.get('smtp_accounts', None)
	if accounts is None:
	raise ValueError('"smtp_accounts" config value is required to send email.')
	if not accounts:
	raise ValueError('at least one account is required to send email.')