invenio 2.0 alpha

* dynamic PEP440 version number (_à la_ django).
* invenio 2.0 alpha.
* **NB:** `git commit --amend` doesn't change the `Date` attribute. You have to do so: `git commit --amend --date="$(date -R)"` instead.

```console
$ git log -1
commit bc23977f9ef69dc992c9f87b1666443495eba017
Author: Yoan Blanc <yoan.blanc@cern.ch>
Date:   Mon Jul 21 09:22:38 2014 +0200

...
$ python setup.py --fullname
Invenio-2.0a0.dev20140721072239
$ python -c "import verlib; print(verlib.NormalizedVersion('2.0a0.dev20140721072239'))"
2.0a0.dev20140721072239
```