Merge pull request #15 from miracle2k/etcd-env

Support ETCD env variable, analogous to DISCOVERD.
Update CONTRIBUTING.md

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Bump deps

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
client: Add Close method

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
client: Replace Client with interface

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Add support for serving HTTPS with SNI.

Update client example to support adding domains with certificates.