AccsCtrl: add createUserAccount() method (#751).

The Access Control service has a new method:
{{{
Topic createUserAccount(Credentials cred)
}}}

REST API:
{{{
POST /accesscontrol/user_account

{"username": username, "password": password}
}}}

BREAKING CHANGE

One client-side REST client methods renamed:
{{{
dm4c.rest.join_workspace -> dm4c.restc.create_membership
}}}

See #751.