Route53: support "alias" rrs by exposing requirement element EvaluateTargetHealth

This hopefully very straightforward change makes it possible to create "alias resource record sets" from boto, by exposing the EvaluateTargetHealth element, which is mandatory for these records (see http://docs.aws.amazon.com/Route53/latest/APIReference/API_ChangeResourceRecordSets.html#change-rrsets-request-evaluate-target-health)

As one of the provided tests shows, when omitting alias_evaluate_target_health=True|False (or without this change), AWS returns an error:

Request:

    <ChangeResourceRecordSetsRequest xmlns="https://route53.amazonaws.com/doc/2013-04-01/">
    <ChangeBatch>
    <Comment>None</Comment>
    <Changes><Change>
    <Action>UPSERT</Action>
    <ResourceRecordSet>
    <Name>alias.example.com.</Name>
    <Type>A</Type>
    <AliasTarget>
    <HostedZoneId>Z2ZCBXT3F7WTVE</HostedZoneId>
    <DNSName>target.example.com</DNSName>
    </AliasTarget>
    </ResourceRecordSet>
    </Change></Changes>
    </ChangeBatch>
    </ChangeResourceRecordSetsRequest>

Response:

    <ErrorResponse xmlns="https://route53.amazonaws.com/doc/2013-04-01/">
    <Error><Type>Sender</Type><Code>InvalidInput</Code>
    <Message>Invalid XML ; cvc-complex-type.2.4.b: The content of element 'AliasTarget' is not complete.
    One of '{&quot;https://route53.amazonaws.com/doc/2013-04-01/&quot;:EvaluateTargetHealth}' is expected.</Message>
    </Error>
    <RequestId>16f31455-9415-11e3-a090-a3885997c561</RequestId></ErrorResponse>