Setuptools installable

Fixes #3 and a couple of other problems found when trying to "pip install", including a source code syntax error.
This line should probably be removed or at least changed to letsencrypt.py script. Otherwise:

```
$ ~/.local/bin/letsencrypt
Traceback (most recent call last):
  File "/home/jakub/.local/bin/letsencrypt", line 9, in <module>
    load_entry_point('letsencrypt==0.1', 'console_scripts', 'letsencrypt')()
  File "/home/jakub/.local/lib/python2.7/site-packages/pkg_resources.py", line 339, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/home/jakub/.local/lib/python2.7/site-packages/pkg_resources.py", line 2470, in load_entry_point
    return ep.load()
  File "/home/jakub/.local/lib/python2.7/site-packages/pkg_resources.py", line 2189, in load
    raise ImportError("%r has no %r attribute" % (entry, attr))
ImportError: <module 'letsencrypt.client.client' from '/home/jakub/dev/letsencrypt/lets-encrypt-preview/letsencrypt/client/client.pyc'> has no 'authenticate' attribute
```