Improved conversion of ndb.KeyProperty to GraphQL

[ekampf]

Basically we want to allow 2 ways of accessing a key's values.

1. Access the key's actual value without querying NDB - we'll just use key.urlsafe()
2. Resolve the key to an entity and query that entity - key.get()

We support 2 naming conventions:

_key or _keys Suffix

NDB Model has a property that looks like this: store_key = ndb.KeyProperty(...)
In this case we'll convert it into the following 2 GraphQL fields:

store_key = graphene.String()  # resolves to entity.store_key.urlsafe()
store = NdbKeyField()  # resolves to entity entity.store_key.get()

Regular Name

NDB Model has a property that looks like this: store = ndb.KeyProperty(...)
In this case we'll convert it into the following 2 GraphQL fields:

store_key = graphene.String() -> resolves to store_key.urlsafe()
store     = NdbKeyField()     -> resolves to entity

[coveralls]

Coverage decreased (-0.1%) to 96.793% when pulling 27422a2 on feature/improved_key_property_behaviour into e75e1be on master.

[coveralls]

Coverage increased (+2.2%) to 99.077% when pulling 0ff360c on feature/improved_key_property_behaviour into e75e1be on master.

[coveralls]

Coverage increased (+2.5%) to 99.388% when pulling d4ba0b9 on feature/improved_key_property_behaviour into e75e1be on master.

[coveralls]

Coverage increased (+2.8%) to 99.694% when pulling 45f493b on feature/improved_key_property_behaviour into e75e1be on master.

[coveralls]

Coverage increased (+2.8%) to 99.694% when pulling a95d6f9 on feature/improved_key_property_behaviour into e75e1be on master.