As part of the Python 3 conversion, the result of dns.zone.to_text()
changed from str to bytes.  This was likely unintentional, as a method
with text in its name should be returning text.

The comment states that relativize must be consistent between
dns.query.xfr() and dns.zone.from_xfr(), and the code fails if they're
not (if check_origin is True, at least).  This means that the rdata is
already correctly relativized (or not).

This also adds a test of creating zones from xfrs, both relativized and
not.

This replaces lots of self.assertTrue() assertions with more specific
assertions, such as replacing assertTrue(x == y) with assertEqual(x, y).

failUnlessRaises was deprecated in py31

failUnless was deprecated in py31

This introduces the same behaviour that BIND has when it encounters implicit TTL
values, namely:
  * The $TTL directive is preferred to set the default TTL [RFC2308].
  * If no $TTL directive is seen prior to the SOA RR, the default TTL is set
    to the minimum TTL field of the SOA RR [RFC1033, RFC1035, RFC1912].
  * If neither $TTL nor an SOA are present, use the last seen explicit TTL on an
    RR or raise a SyntaxError if no explicit TTLs have been seen.

Previously, when neither $TTL nor SOA were present, the TTL for an RR without
an explicit TTL would be set to 0 which is known to be a somewhat dangerous
value and shouldn't be encouraged as a default.

One test seems to rely on this default TTL=0 behaviour whilst testing something
seemingly unrelated (RR starting with whitespace), so that test has been
updated to match this new expectation.

Fixes #254.