Fixes #108.

When we run `init`, currently we generate a simple environment called
'default' with no URI. A better idea is to generate the URI from the
current context of the active kubeconfig file, if it exists.

'env set <name>' sets environment fields such as the name, and cluster
URI. It currently accepts the flags '--name' and '--uri'. Changing the
name of an environment will also update the directory structure in
'environments'.

This commit will implement the first version of the `init` subcommand.
This subcommand initializes a ksonnet application, including generating
the default directory tree and generating the appropriate ksonnet-lib.