Minor memoizing fixes

Resolves https://trac.openmicroscopy.org.uk/ome/ticket/12510 and prevents lwf-stubs.jar from being required when creating memo files.

To test, verify that importing and viewing the file from ticket 12510 (http://downloads.openmicroscopy.org/images/u-track/integrins.zip) works in OMERO.  Memo file creation outside of OMERO should now also be possible with only bioformats_package.jar in the classpath and:

```
Memoizer memo = new Memoizer(new ImageReader(), 0, true);
memo.setId("/path/to/file");
memo.close();
memo.setId("/path/to/file");
memo.close();
```

That code with this PR included should produce a .*.bfmemo file in the same directory; without this PR (and with debug logging enabled), that code should throw a ```ClassNotFoundException```.