Even more webstart codesigning changes

The commit logs should explain everything, main points:
* If `http_proxy` and/or `https_proxy` environment variables are defined they will be converted into the form required by Java
* Removed `release-webstart`, add `release-webstart-unsigned` (no signing) and `release-webstart-signed` (self signed). `release-all` calls `release-webstart-unsigned`
* Removed all existing jarsigning code, everything is done using `omero_insight_sign.py` (so `update-version` won't sign, since there's no way for it to know whether it should or not)

Possibly for discussion:
* `omero_insight_sign.py` sits under `docs/hudson` but is called from `build.xml`
* `docs/hudson/OMERO.sh` calls `release-all` to created unsign artifacts, then signs the zip with `omero_insight_sign.py`. I could add a `release-all-signed` target since this would means we're not following the release signing workflow quite so closely.