Adding ability to pull from a private registry for the build image

Added an extra section to the drone yml file to define auth credentials for the docker repo, the credentials are passed via http header if set.  This allows drone to pull a docker image from a registry that requires authentication. 

I am still a bit unsure on the exact setup, thoughts?

This also fixes #245 

```
image: quay.io/epipho/test-pvt:latest

auth:
  email: $$email
  username: $$user
  password: $$pass

script:
  - env

```

Are you sure, that AuthConfiguration should use `Username` and `Password` from `d.Image`? I thought, that `d.Image.Username` will be used in build.go to specify username for running commands.