Put secret inside register to match actual sockethub-client syntax
When giving the config to sockethub-client, the secret should be inside
an object called register, so we might as well store it directly in
that format.
Move irc 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 irc
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.
irc -> irc-credentials
add tests for irc-credentials module
Merge branch 'secret_inside_register' into ahead
Merge branch 'irc_credentials' into ahead
fix irc tests