Add a Statistics class

Note this is a bit different to the spec
(http://wiki.openstack.org/Ceilometer/blueprints/APIv2)

As wsme doen't really like different types been returned from
the same method.

I have:
GET /v2/meters/<meter> - raw samples
GET /v2/meters/<meter>/statistics - for the stats

Signed-off-by: Angus Salkeld <asalkeld@redhat.com>