Null session API
Created index action for kalibro_ranges_controller

- This commit removes the action ranges_of from
  metric_configurations_controller and adds a index action to
  kalibro_ranges_controller that does the same.
- The route moved to /metric_configurations/:id/ranges_of/ to
  /metric_configurations/:id/kalibro_ranges/
Created index action for readings_controller

- This commit removes the action readings_of from
  reading_group_controller and adds a index action to
  readings_controller that does the same.
- The route moved to
  /reading_groups/:id/readings_of/ to
  /reading_groups/:id/readings/
Change metric_configurations_of action to metric_configurations

- Since metric_configuration isn't nested with
  kalibro_configuration, this was done differently that what was
  done with the index actions of kalibro_ranges_controller and
  readings_controller.
- Also added unit test to test the association between
  kalibro_configuration and metric_configuration
Fixes KalibroConfigurationController unit tests expectations
Changed metric to metric snapshot
Fixed dependencies for metric snapshot and renamed its subclasses
MetricConfiguration creation creates a MetricSnapshot as well
MetricConfigurationsController show method and route

Signed off by: Heitor Reis <marcheing@gmail.com>
MetricConfiguration destroy cascades to MetricSnapshot

Signed off by: Heitor Reis <marcheing@gmail.com>
Removed useless code from kalibro_configurations_controller.

Also:
- Adjusted kalibro_configurations_controller tests.
- Changed compound_metric factory to model from CompoundMetricSnapshot.
- Fixed metric configuration tests.
Changed validation metric_snapshot uniqueness for metric configuration.
Accepting nested attributes for metric snapshot.
Removed unnecessary test.
Merge pull request #12 from mezuro/gatekeeper_integration

Gatekeeper integration