Add gemspec
Add support for summaries

This change adds supports for summaries, which accumlate sample values
and return a set of quantile values (0.5, 0.9, 0.99). A couple of
changes and cleanup were necessary, as Summaries are more complex values
than a simpler counter or gauge.
Add script to automatically run and test the rack example
Update rspec syntax
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
Document rack exporter usage in README
Release v0.2.0
Run tests for Ruby 2.0.0