[commhistory-daemon] Encode notification data to avoid binary safety issues

It's unspecified whether the notification system supports storing binary
hint values, but at least with the current implementation it may be
unsafe due to sqlite's text encoding behavior.

The x-commhistoryd-data hint on notifications is now base64 encoded.
Merge pull request #30 from special/base64-data

[commhistory-daemon] Encode notification data to avoid binary safety issues