installation: pip1.6 ready

**NOTE** install it using: `$ pip install -r requirements.txt` and (i)gnore any existing invenio installation during the process.

Related to: https://github.com/inveniosoftware/invenio/pull/1847
Since `invenio-demosite` repository is applicable to `next` branch only, I would go directly for naming the first official release 2.0.0, to keep the overall harmony.  Hence the dev version could be named `2.0.0.dev20140620`.