Move from piston to tastypie

[iksaif]

djangon-piston is not well maintained, and tastypie seems to be more powerfull. Let's move to that instead (and keep 1.0 API for some time).

• http://django-tastypie.readthedocs.org/
• https://github.com/toastdriven/django-tastypie
• http://tastypieapi.org/