Install into a virtualenv

Install gstatsd into a virtualenv to isolate it from other python apps.

Inspired by gds-operations/puppet-graphite#17

This PR adds specs based on the [gds-operations/puppet-module-skeleton](https://github.com/gds-operations/puppet-module-skeleton) pattern, which were dead useful to verify locally that this actually works. Particularly the spec:system specs.

Would appreciate an ok from @samjsharpe, particularly since I've also excised him from the various copyright notices...