Make client compatible to API version 0.0.2

This change makes the ruby client JSON API 0.0.2 compatible. The following changes were necessary

  * Rename type `summary` to `histogram` until API 0.0.3 is released and supported
  * Set correct `Content-Type` header

Minor changes to be compatible with the golang client label naming:

  * Rename `http_request_latency_microseconds` to `http_request_durations_microseconds`
  * Add `method`, `path` and `code` labels

Misc:

  * Add missing tests for middlewares

While protobuf support is still missing, this makes the client fully useable with current prometheus servers. An example prometheus.conf has been added for manual verification, full integration tests will come in the future.

@matttproud