Include generated Protocol Buffer descriptor.

The Protocol Buffer compiler supports generating a machine-readable
descriptor file encoded as a provided Protocol Buffer message type,
which can be used to decode messages that have been encoded with it
after-the-fact.  The generated descriptor also bundles in dependent
message types.

We can use this to perform forensics on old Prometheus clients, if
necessary.
Move protobuf descriptor and add content-type.

- move to static/generated
- set content-type based on extension '.description'
Merge pull request #179 from prometheus/feature/protocol-buffer-descriptor-inclusion

Include generated Protocol Buffer descriptor.