single record add remove

Progress on issue #11 and issue #36.  Adds support for basic record addition and removal within a set.  Implicit RRSet creation and deletion is expected.

Ex. create or add to the rrset `www.denominator.io.`:`A`

```
dnsApi.getResourceRecordSetApiForZone("denominator.io.").add("www.denominator.io.", "A", AData.create("1.1.1.1"));
```

Ex. remove `1.1.1.1` from the rrset `www.denominator.io.`:`A`, deleting the rrset if `1.1.1.1` is the only element.

```
dnsApi.getResourceRecordSetApiForZone("denominator.io.").remove("www.denominator.io.", "A", AData.create("1.1.1.1"));
```