CoreDNS/SkyDNS provider

[istalker2]

This PR adds support for CoreDNS through its etcd middleware.
Because the middleware is backward compatible with SkyDNS this
commit adds support for SkyDNS as well. In fact, new provider
is available under two names in CLI (coredns and skydns).

All interactions with middleware happen through etcd cluster,
whose location (URIs) is specified via --etcd CLI parameter
by default http://localhost:2379).

The provider translates CoreDNS/DkyDNS SRV records to
A/CNAME + optional TXT endpoints, when reading from etcd and
combines A/CNAME with TXT endpoints back into single SRV record
when writing it back.

Also:

• adds github.com/coreos/etcd package to glide.yaml and vendor folder
  because it is used by the provider
• adds "interfaces" packages and moves Registry and Provider interfaces
  declaration there. This is required in order to use registry in
  provider unit tests. Otherwise there going to be circular imports
  because provider UTests would import registry which imports providers
  back.

[linki]

Thanks for the contribution. Very interesting. We'll have a look.

[istalker2]

replaced with #253. Sorry for inconvenience