Implement additional cgroup files of freezer

Since 3.8 freezer subsystem has parent_freezing and self_freezing
cgroup files. The former reports if the parent is freezing and
the latter reports if itself is freezing. They are used for proper
freezing of hierarchical cgroups.
Update README

We now support subsystems and their cgroup files of
kernel 3.8.
Prepare Changelog for 0.5
Generate README and Changelog for PyPI
Release 0.5