[mms-lib] Handle X-Mms-Retrieve-Status in M-Retrieve.conf

It's one of the ways of telling us that the message has expired.
Merge pull request #39 from monich/retrieve_status

Handle X-Mms-Retrieve-Status in M-Retrieve.conf
[mms-engine] Support for persistent per-SIM settings

This commit defines MMSSettings interface but doesn't actually implement
persistent settings. The implementation will follow as soon as it becomes
clear which kind of storage to use.
Merge pull request #38 from monich/mms_settings

Support for persistent per-SIM settings
[mms-lib] Improved handling of expired push notifications