Implement REST API pagination (#409)

See specifications at:
http://wiki.confine-project.eu/arch:rest-api#pagination
Validate user input: page and per_page.
Add Pagination unit tests.
Extend REST API pagination & filtering tests.
Configure CELERYD_GROUP via command option (#306)
Add support for proxy when calling pip (#302)
Clean up old commands.
Fix resources permissions (#558)
Merge branch 'feature409'

Conflicts:
	controller/__init__.py
Log sliver history on the node (#523).
Handle ping NoReverseMatch at /admin/pings/
Remove not useful 'cn_url' from sliver bulk addition node list.
Merge branch 'master' into feature216