Adding documentation in universal getting started guide.
FIX #227
Provide task to stage output in a format sufficient to build Docker images.

This creates a new docker config, and updates the stage task to create
a Dockerfile which can create an image capable of running an application.
It works with basic Java applications, and basic Play 2.2 applications.
Updated Docker code.

Adds dockerPackageMappings which contains mappings specific to Docker build - defaults
to reading from sourceDirectory/docker (overrideable).
Builds mappings from dockerPackageMappings and universal:mappings (overrideable).
Modifies "docker:stage" command to build a Docker context and Dockerfile from
docker:mappings.
Fix sbt scripted test for Docker
Merge pull request #233 from sbt/wip/getting-started-docs

Adding documentation in universal getting started guide.
Fix typos
Bump version
Updated changes for Docker extension to sbt-native-packager

- Trait DockerPlugin should not extend DockerKeys
- publishArtifact in Docker is false - do not generate POM/etc.
- Tasks for stage do not depend on each other.
- Change the CWD in the Docker container to install location.
- Adding Docker-specific mappings happens outside mapGenericFilesToDocker
- Use defaultLinuxInstallLocation instead of defaultDockerInstallLocation
- dockerBaseImage is set outside Docker config, easier to override
ability to specify java server app start up order in debian implemented
Merge pull request #244 from Blaisorblade/patch-1

Fix typos
FIX #211 and #232. Checking $java_args[@] for memory settings, too.
Didn't ammend the last fix
Merge pull request #245 from kardapoltsev/wip/startup-order

ability to specify java server app start up order in debian implemented
startup levels, dependencies in upstart and systemV impl
Merge pull request #246 from sbt/wip/bash-jvm-options

FIX #211 and #232. Checking $java_args[@] for memory settings, too.
default required service for upstart changed to local-filesystems
setting for java server app startup order implemented as strings
Fix release parsing combinator so classifiers are optional.

Reviewed by @muuki88
Merge pull request #248 from kardapoltsev/wip/startup-order

FIX #239. Start run levels, dependencies in upstart and systemV impl
Merge pull request #236 from garycoady/docker

WIP: Provide task to stage output in a format sufficient to build Docker images