Updated README and setup before uploading to PyPI

I've updated the URL to your obsolete Bitbucket repository, and the installation instructions prior to uploading the app to PyPI.

Yes, there is now a [django-addthis](https://pypi.python.org/pypi/django-addthis) package on PyPI, yeah! This is how I created it:

```
./setup.py register
./setup.py sdist upload
```
Let me know your PyPI user name to make you package maintainer/owner.