SOA fixes and add conversion of . to - in pdns.dns_name func

Except for one line in dns_name() which resolves #105, this PR is to resolve the concerns in #76 with regard to the SOA records. 

We now: 

1. Have proper serials that update when floats are associated/released/disassociated
2. Reference the actual name server rather than 'localhost'
3. Does not have redundant SOAs for every tenancy subdomain.

I tested it out in VBox env and it seems to work fine.