Allow case id as well as hash in inbound email processing to autofile emails on cases

See https://civicrm.stackexchange.com/questions/21407/how-to-expose-the-civicase-hashed-value-that-gets-sent-in-subject-when-sending-a

Currently it only accepts a hash, which is not easily known unless they happen to be responding to an email send via the email icon in the case roles section on manage case. That can still stay but also allow emails that have

[case #1234]

in the subject to get processed onto the case id as entered (e.g. 1234).

Patch attached against master.