extracts ksonnet generator to a new package to make it easier to incorporate in new code

Signed-off-by: bryanl <bryanliles@gmail.com>

`metadata.ClusterSpec` represents a specification for an abstract
Kubernetes cluster. For example, `version:1.7.0` represents a Kubernetes
cluster running a build from 1.7.0. This specification is primarily used
to generate ksonnet-lib.

This struct exposes a method, `data` that will retrieve the OpenAPI JSON
that specifies the API for a Kubernetes cluster. Eventually, `data` will
be able to read a file, pull from a URL, pull from a live cluster, or
pull a specific version of the API from the official Kubernetes
repository.

This commit introduces the the last of these options.

Much of the tooling build around ksonnet applications will be powered by
metadata presented in a structured directory format. This is similar in
principle to how Hugo and Rails structure web applications.

This commit will begin the process of introducing a FS-based state
machine that manages this directory structure. Primarily, this involves
introducing:

1. Init routines for the directory structure
2. Routines to search parent directories for a ksonnet application
   (similar to how git does this with repositories)

Initially, the directory structures looks like this:

  app-name/
    .ksonnet/   Metadata for ksonnet
    components/ Top-level Kubernetes objects defining application
    lib/        User-written .libsonnet files
    vendor/     Mixin libraries, prototypes

The `.ksonnet` file marks the application root, making it possible to
search parent directories for the root.

As time continues, more verbs will be introduced to manipulate this
metadata (including, e.g., vendoring dependencies, searching prototypes,
and so on).