Implement create group

[mitruly]

Implement create group method. Resolves #14.

Changes include:

• Fix Group response
• Add create group endpoint

---

Note for reviewers:
Tested the client against an instance of VinylDNS running on localhost using the following:

VinylDNSClient localClient =
    new VinylDNSClientImpl(
        new VinylDNSClientConfig(
            "http://localhost:9000",
            new BasicAWSCredentials("okAccessKey", "okSecretKey")));

VinylDNSResponse<Group> vinylDNSResponse = localClient.createGroup(new CreateGroupRequest("some-group", "some@email",
    Collections.singleton(new MemberId("ok")), Collections.singleton(new MemberId("ok"))));

System.out.println("response: " + vinylDNSResponse);

This produced the following response:

response: VinylDNSSuccessResponse{value=Group{name='some-group', email='some@email', description='null', id='708c813f-e84d-463a-a2b4-8b18cd8bb1d4', created='2019-02-20T22:42:24.000-05:00', status=Active, members=[UserInfo{id='ok', userName='null', firstName='null', lastName='null', email='null', created=null, lockStatus=Unlocked}], admins=[UserInfo{id='ok', userName='null', firstName='null', lastName='null', email='null', created=null, lockStatus=Unlocked}]}, statusCode=200}

[mitruly]

MemberId is required because the create group endpoint expects a collection of {'id': <id>} values, not simply a collection of ids.

[mitruly]

UserInfo is based off of MembershipProtocol from VinylDNS (see: https://github.com/vinyldns/vinyldns/blob/202dcebba406d4a992e1d000e164422ee43dd5ba/modules/api/src/main/scala/vinyldns/api/domain/membership/MembershipProtocol.scala#L71-L79).