pkg/transformer: transformer from dockerfile

Following the [request](https://github.com/mcuadros/rocketizer/issues/1
) from @philips this is a proposal to merge the main features from https://github.com/mcuadros/rocketizer intro `spec`

Currently this not is working at the current version of appc/spec, works with the original ACI format. Another problem could be that the test are based on [gocheck](https://labix.org/gocheck).

The main purpose of this PR is obtain the basic guides to merge it into master, the main goals, etc.

/cc @jonboulle 

https://github.com/appc/spec/issues/15