Add import script

[nichtich]

Import of backend data from TSV file should be executed by a script so we can import the same way in multiple backend (seee #26).

[nichtich]

To also solve #24, the script could have option to purge database content (default, used when importing a fill dump) or to do an incremental update (likely much slower as it requires to query the backend).

[stefandesu]

For some reason, I've been tagging the commits related to this with #24.

There's now a basic working implementation of the import script. Things that are missing:

• Better error handling
• Documentation