stats: added collection of runtime statistics with handystats.

List of added metrics:
- counters:
  - 'thevoid.connections'
  - 'thevoid.active_connections'
  - 'thevoid.requests'
  - 'thevoid.requests.<code>'
  - 'thevoid.requests.<code group>'
  - 'thevoid.requests.<method>'
  - 'thevoid.requests.<method>.<code>'
  - 'thevoid.requests.<method>.<code group>'

- timers:
  - 'thevoid.requests.time'
  - 'thevoid.requests.<code group>.time'
  - 'thevoid.requests.<method>.time'
  - 'thevoid.requests.<method>.<code group>.time'

Here
- <code> is request's status code (e.g., 200, 404)
- <code group> is request's status code group (e.g., 2xx, 4xx)
- <method> is request's method (e.g., GET, POST)