Add requirements.txt for all dependencies

I added a ``requirements.txt`` file in case a user does not have all the necessary python modules installed. To verify this covers all dependencies, I ran it under a clean environment using virtualenv and exported all installed modules:

```
$: virtualenv env
$: source env/bin/activate
$: pip install docopt
$: pip install pycrypto
$: pip install requests
$: pip freeze > requirements.txt
```

Now ``pip install -r requirements.txt`` takes care of all dependencies.