When the user calls `ks init <whatever>`, we need to emit an `app.yaml`
for the new project. This commit will introduce such behavior.

`ks env add myenv --context=dev` will initialize the environment URI
and namespace based on the context `dev`, as specfied in the
kubeconfig file. The same applies for `ks init foo --context=dev`.

If both the context and uri argument is not provided, the current
context is used.

This change will also remove uri as a mandatory arg in `ks env add`.
The uri will be moved to a flag. It cannot be used at the same time as
`context`.

Environments currently have the concept of a server URI, but it is
ambiguous which cluster namespace to use.

This commit will introduce the concept of namespaces to the env
commands.

For example,
`kubecfg env add staging http://mock-staging-uri \
 --namespace=staging-namespace`
`kubecfg env set staging --namespace=staging-namespace`

The default environment will use the namespace of the default context.

This commit will also update commands that take the <env> arg such as
`apply` to make use of the env namespace, if specified.

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.