Selenium2 update 11246

This updates the webadmin selenium tests and the webgateway/test/seleniumbase.py to use Selenium-2.

To test:

Install Selenium-2:
```
$ pip install selenium
```
NB: we previously shipped selenium with OMERO. Do we want to continue to do that?

Install Firefox and Chrome browsers
Install Chrome webdriver: download from https://code.google.com/p/chromedriver/downloads/list and put on your PATH.

Start OMERO.web running at localhost

Run tests on Firefox and Chrome.
```
omero web seleniumtest webadmin localhost http://localhost:4080 firefox
omero web seleniumtest webadmin localhost http://localhost:4080 chrome
```
4 tests should pass!