Ability to start HTTP server

This changes allow developers to start HTTP server while running Firefox to test the addon.
If a binding port number is provided, e.g.:
```
./build.py -b 8000 run
```
the integrated Python HTTP server will be started along with Firefox and the browser will automatically open the page of `tests` folder (http://localhost:8000/tests), which is quite convenient to test with provided json files.

The url can be specified manually as well:
```
./build.py -u localhost:8080 run
```

The getopt is also replaced by argparse (requires Python 2.7).