Selenium webclient setup 11314

NB: this PR is on top of https://github.com/openmicroscopy/openmicroscopy/pull/1340

In order to prepare for a whole bunch of webclient tests, I have tried to provide an easy way to create a new user in a new group and login as that user, all in a single call that can be used at the beginning of tests. Similar to integration tests ```new_client_and_user()```.

I have therefore moved the existing login(), createExperimenter() and createGroup() from the webadmin tests to the seleniumBase class. Also, I have tried to improve the speed of these:

Instead of submitting html forms to do login / createGroup / createExperimenter, I simply use jQuery $.post() to the appropriate url. This allows all these actions to be carried out on any page that has jQuery (E.g. the login page).

To test:
 I have created a webclient selenium test stub that simply creates a new user in a new group and logs in as that user. We visit the /webclient/ page for 5 seconds so you can see this is successful, but don't do anything else at this point.
```
$ omero web seleniumtest webclient localhost http://localhost:4080 firefox
```

Also test:
Need to check that the login(), createGroup() and createExperimenter() methods used by the webadmin tests are still behaving as expected:
```
$ omero web seleniumtest webadmin localhost http://localhost:4080 firefox
```