Install to Python virtualenv

Install Graphite, Carbon, Whisper, and (nearly) all dependencies into a
virtualenv using the `stankevich/python` module. This retains the same
sandboxing that `--install-option`/`--prefix` was doing, but with the added
benefit of:

- Allowing the version param to upgrade and downgrade the packages without
  writing the `pip` logic ourselves.
- Isolating dependencies from the system's Python/pip paths.

Absolute binary paths are used to reference the virtualenv without needing
to `source bin/activate` each time. Cairo is shimmed into the virtualenv
from the system package because of the reasons described in the class doc.
I've rebased the dependencies against Graphite's `requirements.txt` to
reference specific versions and dropped:

- LDAP support: because it has compile dependencies and I'm not aware of
  anyone currently using this functionality from the module.
- pysqlite2: because Python >=2.6 has this builtin.

I'm using an earlier version of the `python` module because of the following
bug which causes the virtualenv to be recreated on every run:

- stankevich/puppet-python#46

---

This contains/depends on test refactoring in #16 - so should **not** be merged before that.

/cc @rjw1 @KushalP @philandstuff