Support attributes in JsonTask

Adds all children of the 'attributes' field in the JSON
returned by the subprocess to the protobuff sent to riemann.

Adds the attrprefix before keys.
Fix test import.
Merge pull request #6 from samn/json-task-attributes

Support attributes in JsonTask
Only strip units from attributes when converting to metric.
Merge branch 'master' of github.com:bmhatfield/riemann-sumd