Webstart access limited

This PR allow configuring access to WEBSTART. Configurable as follow:
 - ```omero.web.webstart_admins_only (true, not_set or false)``` it allow to limit access to WEBSTART to only admins and group owners.

In order to test this PR:
- set ```bin/omero config set omero.web.webstart_admins_only "true"```, and then:
 - log out and go to ```http://host/webstart/```, check if you are redirected to the login page when accessing ```http://host/webstart/```
 - log in as ```root``` and ``group_owner``` and check if ![icon-omero-insight-toolbar](https://cloud.githubusercontent.com/assets/1065155/2705352/b1c1053e-c47c-11e3-962d-29c4c7bbb5c4.png) is present and you can start insight by clicking on ![icon-omero-insight](https://cloud.githubusercontent.com/assets/1065155/2705370/24123374-c47d-11e3-8061-7a0bb33f3b3e.png).
 - log in as ```user``` and see if ![icon-omero-insight-toolbar](https://cloud.githubusercontent.com/assets/1065155/2705352/b1c1053e-c47c-11e3-962d-29c4c7bbb5c4.png) is unavailable. Go to ```http://host/webstart/``` and check if there is a message ```Webstart is not available```
- unset ```bin/omero config set omero.web.webstart_admins_only```, and then:
 - log out and go to ```http://host/webstart/```, check if you can start webstart,
 - log in as ```root```, ``group_owner``` and ```user``` and check if ![icon-omero-insight-toolbar](https://cloud.githubusercontent.com/assets/1065155/2705352/b1c1053e-c47c-11e3-962d-29c4c7bbb5c4.png) is present and you can start insight by clicking on ![icon-omero-insight](https://cloud.githubusercontent.com/assets/1065155/2705370/24123374-c47d-11e3-8061-7a0bb33f3b3e.png).