Model manager changes

[csantero]

Summary of changes:

1. ModelManager now discovers everything it needs to know about a type's properties when that type is registered, rather than when the type is first (de)serialized. Closes ModelManager caches should be populated at registration time #82
2. GetProperties() now returns a wrapper class for PropertyInfo called ModelProperty, which stores the PropertyInfo as well as the serialized json key and whether the property should be ignored by default in serialization. This class is abstract and has two child classes: FieldModelProperty and RelationshipModelProperty. The latter tracks the cardinality of the relationship as well as the related type.
3. GetPropertyForJsonKey() returns a ModelProperty.
4. GetJsonKeyForProperty is gone from IModelManager. Any code that was using this can use the field on ModelProperty instead.
5. You can now override a property's json key by adding a [JsonProperty] attribute. Closes Allow overriding a property's serialization json key #87
6. Attempting to filter by an unknown field will return a 400 error, rather than performing no filter.
7. Models that use [UseAsId] to specify the id property no longer serialize the property both as id and as the property's formatted name. I feel that this is a bad practice due to not being compatible with a 1:1 mapping of json key to property, and would require special handling to make work given these new changes. Furthermore, ember data would probably squawk if you change a regular field and the server's response comes back with a new id. If a user really wants this they can accomplish it by adding a surrogate Id field in their model class, removing [UseAsId], and ensuring that the two are the same.

This PR should improve performance and make finding bugs in one's models easier, by moving several calculations to registration time that previously didn't happen until requests are made. It also puts the model manager in charge of some concerns that used to be in the formatter's domain, but really ought to be part of the registration process. Most significantly, the formatter no longer needs to figure out whether a property is for a field or a relationship every time that property has to be serialized.

@SphtKr Do you have any feedback before I merge? I have some more work I want to get in that depends on this, but I want to make sure this meets your approval first.