Add support for SCRAM-SHA1

Expected behaviour

User should be able to select SCRAM-SHA1 as a form of authentication for IMAP.

Rationale

CRAM-MD5 is deprecated. SCRAM-SHA1 is fairly well adopted. If SSL/TLS is deployed we don't gain much security, but SSL/TLS isn't everywhere and CRAM-MD5 is obsolete.

Issues arising

The algorithm is more complex than others. It involves generating random nonces and the actual schema is quite difficult to follow. Deployment is less common so it's more complex to test but the original requester did offer a temporary test host.

On a UX level we don't actually expose the encryption method. We allow them to select from 'Plain', 'Encrypted' and 'Client certificate'.

According to the code documentation :

PLAIN and CRAM_MD5 originally referred to specific SASL authentication
* mechanisms. Their meaning has since been broadened to mean authentication
* with unencrypted and encrypted passwords, respectively.

In theory this is nice. In practice it's a white lie - it does translate to that in the UI but we don't support any encryption except CRAM_MD5.

Source of request:

Google Groups Feedback https://groups.google.com/forum/#!topic/k-9-mail/UHhXyRY74SU

The issue names no files or tests. Start by locating the IMAP authentication entry point and reviewing the existing CRAM-MD5 handling; done means users can select and use SCRAM-SHA1 for IMAP, with a way to test the mechanism.