Continuous integration of browser tests

New version of #51 with clean log.

Travis configured to download and install selenium server and run browser tests using the version of Firefox they ship.

New `make selenium` has been added as well as a `selenium.sh` init script.

`make selenium` will: 

- download a selenium server binary if it has not been done already
- start the selenium server
- run the browser tests
- stop the selenium server

The `selenium.sh` script has the following commands available:

- `install` installs the server
- `start` starts the server
- `stop` stops the server
- `reload` reloads the server