Add Validations to model

[angrifel]

We should allow validations to occur on the model at different levels.

• at the field level

entities:
  entity_1:
    ...
    members:
      field_1: ...
      field_2:
        type: ...
        validations: 
          validation_name_1 : [validation rule here]
          validation_name_2 : [validation rule here]
          #other validations here
      #other fields here
  entity_2: ...
  #other entities here

• at the entity level

entities:
  entity_1:  ...
    members: ...
    validations: 
      validation_name_1 : [validation rule here]
      validation_name_2 : [validation rule here]
      #other validations here
  entity_2: ...
  #other entities here

• at the model level.

enums: ...

entities: ...

validations: 
  validation_name_1 : [validation rule here]
  validation_name_2 : [validation rule here]
  #other validations here