User mapping feature

[giraldeau]

Allow mapping of users between instances based on emails.

I tested the function modifyUserMapping and it seems to be okay, but I did not test the complete migration script. It should work, but before merging I would do a full test later this week.

Your comments are welcome. Thanks!

Fixes #5

[giraldeau]

Fix #5

[giraldeau]

Actually, we have to delete workDir, otherwise we might create an archive with unrelated content from previous importation.