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