support for route53 aliases

This adds support for route53 aliases, a dependency of #244.  Aliases look like normal `A` or `AAAA` record sets, except that the values are instances of `AliasTarget` instead of `AData` or `AAAAData`.

ex.
```java
ResourceRecordSet.<AliasTarget> builder()//
                .name("fooo.myzone.com.")//
                .type("A")//
                .add(AliasTarget.create("Z3I0BTR7N27QRM", "ipv4-route53recordsetlivetest.adrianc.myzone.com."))//
                .build();
```

This also adds a cli hook to create ELB aliases, using the `--elb-dnsname` flag.

Ex.
```bash
$ denominator -n route53 record -z myzone.com. add -t A -n foo.myzone.com. --elb-dnsname abadmin-795710131.us-east-1.elb.amazonaws.com
;; in zone myzone.com. adding to rrset foo.myzone.com. A values: [{HostedZoneId=Z3DZXE0Q79N41H, DNSName=abadmin-795710131.us-east-1.elb.amazonaws.com}]
[Route53#listHostedZones] ---> GET https://route53.amazonaws.com/2012-12-12/hostedzone HTTP/1.1
[Route53#listHostedZones] <--- HTTP/1.1 200 OK (623ms)
[Route53#listResourceRecordSets] ---> GET https://route53.amazonaws.com/2012-12-12/hostedzone/Z3I0BTR7N27QRM/rrset?name=foo.myzone.com.&type=A HTTP/1.1
[Route53#listResourceRecordSets] <--- HTTP/1.1 200 OK (161ms)
[Route53#listResourceRecordSets] ---> GET https://route53.amazonaws.com/2012-12-12/hostedzone/Z3I0BTR7N27QRM/rrset?name=foo.myzone.com.&type=A HTTP/1.1
[Route53#listResourceRecordSets] <--- HTTP/1.1 200 OK (146ms)
[Route53#changeResourceRecordSets] ---> POST https://route53.amazonaws.com/2012-12-12/hostedzone/Z3I0BTR7N27QRM/rrset HTTP/1.1
[Route53#changeResourceRecordSets] <--- HTTP/1.1 200 OK (152ms)
;; ok
```