Add executable for selenium-server-standalone jar

Currently, this is how selenium is run, which seem a little unwieldy:

```
==> Caveats
To have launchd start selenium-server-standalone at login:
    ln -sfv /usr/local/opt/selenium-server-standalone/*.plist ~/Library/LaunchAgents
Then to load selenium-server-standalone now:
    launchctl load ~/Library/LaunchAgents/homebrew.mxcl.selenium-server-standalone.plist
Or, if you don't want/need launchctl, you can just run:
    java -jar /usr/local/opt/selenium-server-standalone/selenium-server-standalone-2.39.0.jar -p 4444
```

Would be great if we could just run this:

```sh
selenium-server-standalone -p 4444
```