Add AAAA Zone record functions and tests

Add the same abilities as 'A' records for 'AAAA' to support IPv6 in Route 53. All functions and tests are essentially copied from the existing 'A' functions, such as get_a -> get_aaaa, delete_a -> delete_aaaa.

Integration tests are also included and results shown below:
```
[ematthews@frederick route53]$ python test_zone.py
test_a (__main__.TestRoute53Zone) ... ok
test_aaaa (__main__.TestRoute53Zone) ... ok
test_cname (__main__.TestRoute53Zone) ... ok
test_get_nameservers (__main__.TestRoute53Zone) ... ok
test_get_records (__main__.TestRoute53Zone) ... ok
test_get_zones (__main__.TestRoute53Zone) ... ok
test_identifiers_lbrs (__main__.TestRoute53Zone) ... ok
test_identifiers_wrrs (__main__.TestRoute53Zone) ... ok
test_mx (__main__.TestRoute53Zone) ... ok
test_nameservers (__main__.TestRoute53Zone) ... ok
test_toomany_exception (__main__.TestRoute53Zone) ... ok

----------------------------------------------------------------------
Ran 11 tests in 4.216s

OK
```