DO NOT MERGE: new pip install procedure

* To install Invenio run:
  > pip install --index-url=http://invenio-software.org/download/python/simple/ -r requirements.txt

 For extra features run:
  > pip install --index-url=http://invenio-software.org/download/python/simple/ -e .[extras]

Tested on local and works fine (note that I am skipping `python setup.py develop`).