[nemo-qml-plugin-contacts] Store avatar with MD5 filename

Derive filename from MD5 hash of content rather than use a random
UUID value.
[nemo-qml-plugin-contacts] Store device-local avatar data to cache

If a VCF contact specifies a PHOTO property via a local path, import
the data read from path into a cached avatar.
Merge pull request #52 from matthewvogt/avatar-name

[nemo-qml-plugin-contacts] Store device-local avatar data to cache