Add docker.io Drone plugin which creates a docker image for each successful build

This change adds a new Drone plugin for creating a docker image. The plugin
creates a docker image for the test build and pushes the image to images.docker.io.
Add tests for docker plugin

These tests just check that the docker commands show up in the buildfile
Merge pull request #1 from Clever/docker-plugin

Drone-docker plugin
Write login command silently so it doesn't print password
Only grab the last part of the docker repo name
Also publish "latest" image
Merge pull request #3 from Clever/publish-latest

Also publish "latest" image
Allow publishing from base directory

Some our our respositories have Dockerfiles with commands like ADD . /path. These don't
work if we do - < Dockerfile because the only thing in "." will be the Dockerfile, not
the other things we want. This allows us to support that use case.
Merge pull request #4 from Clever/publish-from-base-dir

Allow publishing from base directory
Tweak command to install docker per upstream comments
Merge pull request #5 from Clever/update-docker-client-install

Update command to install docker per upstream comments
Add image for Go 1.3
Updates README to mention go 1.3
Merge pull request #6 from Clever/add-go1.3

Add image for Go 1.3
Derp when updating the docker install command
Merge pull request #7 from Clever/tweak-docker-client-install

Derp when updating the docker install command
Private Repository Support
Fixing indents, adding repo_name option and additional tests
Simplified endpoint login behaviour
Merge pull request #8 from daviddyball/master

Private Repository Support