Commit adds:
* Implementation of PowerDNS as a provider
* Tests for said implementation
* github.com/ffledgling/pdns-go, which provides go client bindings for
  PowerDNS's HTTP API, as a dependency
* "pdns" as an additional option for the `--provider` flag
* `--pdns-server` and `--pdns-api-key` as additional flags for PowerDNS
  specific configuration

* add "dyn" provider
* add several --dyn-* args to configure Dyn login
* add github.com/nesv/go-dynect/dynect@0.6.0 to Gopkg and vender/ (the client
  of choice by Terraform)
* make externdns.Version public so it can be stored when committing
  zone changes
* add tutorial for Ingress resources and update root README.md file

Dyn REST API is documented here: https://help.dyn.com/dns-api-knowledge-base/

Example usage:

  external-dns \
    --provider=dyn \
    --dyn-customer-name=acme \
    --dyn-username=acme-api \
    --dyn-password=t0pS3cr3t \
    --domain-filter=portal.acme.com \
    --zone-id-filter=acme.com \
    --namespace=my-test-ns \
    --log-level=debug \
    --txt-prefix=_