Model validations for Kalibro Configuration.

Also removed unnecessary method.
Model validations for Reading Group.
Model validations for Reading.
Model validations for Metric Configuration.

Created association with Metric.
WIP: Model validations for Metric.

What is left to be done?
- Test validations with conditionals.
- Validates uniqueness of code in the scope of a kalibro_configuration,
  not in the scope of a metric_configuration.
Added validator on MetricConfiguration to check code uniqueness

Signed off by: Heitor Reis <marcheing@gmail.com>
Added validators on KalibroRanges

Signed off by: Heitor Reis <marcheing@gmail.com>
Added dependency validators

Signed off by: Heitor Reis <marcheing@gmail.com>
Cascade destroy for CompoundMetrics when destroying MetricConfiguration.
Merge pull request #10 from mezuro/model_validations

Model validations
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