[contactsd] Add contacts DB exporter plugin

Create a plugin which listens for changes in the privileged contacts
database, and exports those changes to a nonprivileged database.

The nonprivileged database contains a simplified view of the contacts
on the device, which can be accessed by any application, since
restricted information is excluded from the export.

Changes made in the nonprivileged database are promoted to the
privileged database.
[contactsd] Create nonprivileged links to privileged avatar images

When exporting a privileged contact to the nonprivileged database,
if it has any avatar details that refer to privileged image paths,
then create nonprivileged links to those files are update the avatar
detail to refer to the accessible path.
Merge pull request #62 from matthewvogt/exporter

[contactsd] Add contacts DB exporter plugin