Webstart tsa conditional

Make timestamping optional
See #2087, #2069

Standard build as before: `docs/hudson/OMERO.sh`

Using password-file and timestamping:
```
./build.py release-webstart \
    -Djarsign.keystore=/Volumes/ome/team/simon/code-signing/store.jks \
    -Djarsign.alias=testcodesign \
    -Djarsign.storepassfile=/Volumes/ome/team/simon/code-signing/store.passwd
    -Djarsign.tsa=http://time.certum.pl
```
Omit `jarsign.tsa` to disable timestamping.
Omit `jarsign.storepassfile` to use a password (Java 1.6 compatible), optionally set `jarsign.storepass`.

See `jarsign.*` in `local.properties.example`.