Metadata reimport

This PR allows you to re-import metadata when BF readers are updated.
To test: import an image then alter any of the following objects:

```
Image 
  ├── Pixels
        ├── Channel ── LogicalChannel ── LightSettings
        └── Channel ── LogicalChannel ── DetectorSettings
  ├── Instrument
  └── ObjectiveSettings ── Objective

```

Then run:
```
$ bin/omero import --reimport FILESET_ID
...
2014-10-08 14:55:14,980 4437       [l.Client-1] INFO   ormats.importer.cli.LoggingImportMonitor - METADATA_REIMPORTED Step: 1 of 2  Logfile: 908
2014-10-08 14:55:15,012 4469       [l.Client-0] INFO   ormats.importer.cli.LoggingImportMonitor - OBJECTS_RETURNED Step: 2 of 2  Logfile: 908
2014-10-08 14:55:15,143 4600       [l.Client-1] INFO   ormats.importer.cli.LoggingImportMonitor - IMPORT_DONE Imported file: .../2014-10/01/09-53-12.711
...
Fileset:502
Image:502
2014-10-08 14:55:15,143 4600       [l.Client-1] INFO      ome.formats.importer.cli.ErrorHandler - Number of errors: 0
Reimport succesful.
```

and see if it was repopulated. Check logfile.


I suppose it is possible that the hasher property may have got set to `null` at some point or the server may no longer be able to provide some previously existing checksum algorithm? Both each seem unlikely, to be honest, but *if* there's an easy way to cover such cases then that might be nice. (For now it might suffice if runtime exceptions are also adequately logged, which perhaps they are already.)