Implement getGroup

[mitruly]

Implement getGroup method. Resolves #17.

Changes include:

• Implement getGroup
• Add tests

---

Note to reviewers: Tested this against a local instance of VinylDNS:

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

VinylDNSResponse<Group> vinylDNSResponse = localClient.getGroup(new GetGroupRequest("140e108d-0b52-4030-8958-c210ceda9997"));

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

This resulted in the following response:

response: VinylDNSSuccessResponse{value=Group{name='fsd', email='sdfsd', description='null', id='140e108d-0b52-4030-8958-c210ceda9997', created='2019-02-24T18:49:31.000-05:00', status=Active, memberIds=null, adminUserIds=null}, statusCode=200}