Merge Runner: merge the image(s) and annotation(s)

[bobslee]

Since the PR #15, the image and annotation elements won't be merged yet.
See for initial discussion/info the PR #15

The code which skips those elements is in merge() function of the runner.py, as:
if tag not in ['annotation', 'image']:

This still needs to be implemented (with tests) and reviewed.
Ideally to develop in conjunction with the refactoring of the merge, reported in issue #12