Add methods for TXT and AAAA records similar to existing MX/A records

Existing:
```
r53 = boto.connect_route53()
zone = r53.get_zone("mysite.internets")
zone.get_a("host.mysite.internets")
zone.get_mx("mysite.internets")
zone.find_records("TXT", "host.mysite.internets")
```

This PR adds
```
zone.get_txt("txt.mysite.internets")
zone.get_aaaa("ipv6host.mysite.internets")
```

Also adds relevant update/delete/add methods.