Validate schemas when Model constructor is invoked

[agarzola]

When a CustomModel is created, the object passed in to the Model constructor should be validated. Criteria should be as follows:

• type and endpoint must be strings and cannot be missing.
• endpoint should start with a / and have only URL-safe characters.
• Schema object should not have any property names that collide with Jsonmonger’s interface:
  • built-in method and property names: fetch, save, destroy, toObject, saved
  • common internal property names, such as __schema, __changed, etc.
  • It might make sense to validate this programmatically, so that new internal properties or built-ins are automatically reserved.
• Properties pointing at relationships should be checked for depth. relationships.field_name is correct; relationships.field_name.data is not.