install: 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]

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>