Support versioned telemetry providers.

client_golang was updated to support full label-oriented telemetry,
which introduced interface incompatibilities with the previous
version of Prometheus.  To alleviate this, a general fetching and
processing dispatching system has been created, which discriminates
and processes according to the version of input.
Merge pull request #48 from matttproud/feature/versioned-processors

Replacing the Previous …