Use google-cloud-dns instead of google-cloud for GoogleCloud

On 2018-06-18, google-cloud package was deprecated and so
it requires us to use google-cloud-dns instead of google-cloud
for GoogleCloudProvider (Google Cloud DNS).

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

add AXFR source to OctoDNS

Adds a new source requested in #239. This source allows a user to pull data from
a legacy system (Bind9, etc.) that does not have an API/existing provider via
AXFR Zone Transfer.

Implement EtcHostsProvider, content to be used in /etc/hosts

Fix minor typo in contributing doc, layout

if user does not explicitly set Access Key ID and Secret Access Key t…

change all answers from NS1 to absolute for types that require it

boto3's methods as fallback

...for testing or emergencies

Lenient add record

Apply sub-zones before parents

0.9.1 version bump and changelog update

README.md: your -> you're typo

Include info about provider specific reqs in README

Revert the setup.cfg infra and go back to requirements.txt

I've continually run into problems with the setup.cfg route that I'm not
interested in investing the time to debug and fix. Looking around it doesn't
seem to be very common yet so I've had a hard time finding help/docs etc.

pycodestyle updates, mostly escaping \, couple r's, ignore break on binary

Skip unsupported records for NS1 & DNSimple

Mark SSHFP algorithm 4 as valid

Implements the unsupported records skipping fix [proposed by ross](https://github.com/github/octodns/issues/176#issuecomment-359294960) in #176 for DNSimple and additionally for NS1.

Fixes #176 and also the NS1 version of it (`AttributeError: 'Ns1Provider' object has no attribute '_data_for_DNSKEY'`) that currently affect domains that have DNSSEC enabled.

[RFC 7479](https://tools.ietf.org/html/rfc7479) introduces support for Ed25519 host keys for the SSHFP record.