Add reference object to Response

[christopheranderson]

Problem:
.get is confusing, but we need a way to get a reference to a given resource. We decided on a combo approach of renaming .get and adding the reference to the response of async calls.
From #39:

4. Return reference object in response from async operations.
   - Example: const { resource: db} = client.databases.create({id: foo});
   - Pros: Helps reduce number of lines of code for many scenarios (like db and container create)
   - Cons: Doesn't remove need for .get because we don't want to force an async operation just to get a reference. Won't work with Steve's idea for simplifying the response types.

Additionally, we'll rename result to body to

Implementation details:

interface CosmosResponse<T, U> {
    body: T;
    headers: Headers;
    ref: U;
}

interface DatabaseResponse extends CosmosResponse<DatabaseDefinition, Database> {
    body: DatabaseDefinition;
    headers: Headers;
    ref: Database;
    database: Database;
}

/* Similar pattern for other types */

[christopheranderson]

Question: What should be done about QueryIterator?

[christopheranderson]

Closed in #52