Implement nested_inlines

Just do:
pip install -e git+git://github.com/Soaa-/django-nested-inlines.git#egg=django-nested-inlines
cp reception/media/js/nested.js src/django-nested-inlines/nested_inlines/static/admin/js/

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Fix case of reservation without apartment

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Fix availability!!!!

Finally you can find available apartments. Do not use complex
queries. Just parse apartments and their reservations.

Plus add a method to get datetime objects from html datefields
(e.g. 2013-2-23)

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>