Use Content-Type data for telemetry versioning

ProcessorForRequestHeader now looks first for a header like
`Content-Type: application/json; schema="prometheus/telemetry";
version="0.0.1"` before falling back to checking
`X-Prometheus-API-Version`.
Add version 0.0.2 processor
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
Merge pull request #192 from prometheus/feature/negotiate-telemetry-schema-through-mime-type

Use Content-Type data for telemetry versioning