Fix unicity of Online objects.

The queryset method get_or_create() does not ensure unicity by itself.
Its args must be set as unique as database level to ensure atomicity,
as stated by official django documentation.

When a duplicate is inserted in the database, all subsequent call to
get_or_create() with this ident asserts so all requests from the
corresponding user/ip ends up with HTTP error 500.
Merge pull request #1 from gutard/fix_duplicates

Fix unicity of Online objects.