Dependency on Beautiful Soup 4

The Dependency of this package on [Beautiful Soup 4](https://pypi.python.org/pypi/beautifulsoup4/) is not declared in its `setup.py` file, so `easy_install` won't install that required package, leading to errors like this:

```
Traceback (most recent call last):
File "…", line 3, in
from pyrobot.browser import RoboBrowser
File "…/pyrobot-0.1.0-py3.3.egg/pyrobot/__init__.py", line 3, in
from .browser import RoboBrowser
File "…/pyrobot-0.1.0-py3.3.egg/pyrobot/browser.py", line 7, in
from bs4 import BeautifulSoup
ImportError: No module named 'bs4'
```