Provide Discriminator For Protocol Buf. Streams

This commit introduces an incomplete processor that decodes varint
record length-delimited streams of io.prometheus.client.MetricFamily
Protocol Buffer messages.  The Go client presently does not support
generation of said messages, but this will unblock a number of Java
changes.
Implement Protocol Buffer Stream Decoding

This commit introduces protocol buffer telemetric stream decoding.
Merge pull request #21 from prometheus/feature/accept-header-discrimination

Support Protocol Buffer Message Decoding