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
have we found a consensus about numbering scheme?

1. `0.1dev1`
1. `0.1.dev1`
1. `0.0.50`