Reading color should be a string
Sets reading_group_id on Reading creation
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>