Refactor processor for 0.0.2 schema

Primary changes:

* Strictly typed unmarshalling of metric values
* Schema types are contained by the processor (no "type entity002")

Minor changes:

* Added ProcessorFunc type for expressing processors as simple
  functions.
* Added non-destructive `Merge` method to `model.LabelSet`
Add LabelSet helper to fix go 1.0.3 build
Merge pull request #195 from prometheus/refactor/processor002

Refactor processor for 0.0.2 schema