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

This tells us whether the sender requested a read report.
[mms-lib] Better type checking
Merge pull request #37 from monich/types

Better type checking
Merge pull request #36 from monich/read_report_req

Handle X-Mms-Read-Report in M-Retrieve.conf
[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.