Move email credentials into CredentialsStore, update schema
It is better to have credentials as a separate OAuth scope, and to
be able to encrypt them with a master password. So far only sockethub
credentials were in a separate module, this PR does the same for email
credentials, which were tagged onto the messages module until now.

This commit also switches to the new CredentialsStore class, and updates
the schema to match the latest API changes in sockethub.
remove messages store
In meute 0.4, messages were stored in the email module. From meute 0.5
onward, the messages module from Dogtalk will be used, so this
functionality is no longer needed inside the email module
email -> email-credentials
unit tests for email-credentials
Merge branch 'email-credentials' into ahead

Conflicts:
	src/messages.js
fix tests for email