#23 conflict resolution

[Rob Helmer]

we should be able to detect conflicts and to guide the
user through the situation, giving them some choices like

* take the new version ( overriding my changes ) and
back up my version ( default )

* keep my version and back up the old version

* show me the diff between the two versions

* upload a new version that will supersede both

Also, every mention of the new and old versions should
be an HREF to that version, so it can be downloaded,
put together and re-uploaded.