add memory cgroup
set memoty_cgroup disable by default
Merge pull request #247 from action-io/memory-cgroup

Measure memory usages of cgroups
Fixes #246, this changes publish_counter and derivative to default to not allowing negative return values. I believe this is what most people will want. You can over ride this with allow_negative=True. This fixes the issue globally.
Update changelog
pep8
Fixes #243, this renames the xen collector to allow it to work properly with python 2.4