installation: pip1.6 ready setup.py

Addressing some of the concerns from #1797 by making setup.py a bit simpler.

* Removes the `dependency_links` from `setup.py`.
* Removes the many parsing and regexes of requirements files content.
* Relies on `requirements.txt` to install non-pypi packages.

```
# Don't do this
$ python setup.py develop
# Instead do that
$ pip install -r requirements.txt
```
At term, the goal is to have `pip install invenio`. It means that all the dependencies will have to be available from the cheeseshop by the time pip 1.6 is released.