Don't crash on runner exit after soft-stop

When issuing a soft-stop (either directly or via soft-restart) the
runner registered an event handler for 'exit' with the callback that
is given. Because that callback eventually bubbles up to a beforeCreate
hook on a ServerAction model instance, a phantom response is sent at
any point in the future when the runner emits an 'exit' event. This
manifests as Express throwing an exception because the callback
ultimately attempts to send a response, complete with new response
headers, on a response that has already been sent.

This went unnoticed because the tests did not trigger any restarts
after the soft-restart test and because there is no soft-stop test.
v1.3.0
Merge branch 'release/1.3.0' into production
Merge tag '1.3.0'

v1.3.0
test: Upgrade node in Vagrant test VM
don't copy /etc/skel in service installation

When creating a system user for strong-pm the home directory should
not by populated with the contents of /etc/skel like a normal user.
Store/load persisted environments for applications

* Environment is loaded from the Server instance and used for prepare
  and run.
* Attach environment to commit object, since it can change with each
  commit.
* Add /env to test app
pmctl: Add env-set to configure app env vars

Restart goes through the existing replace mechanism which has been
updated to test for changes in environment. If the environment
variables of the replacement do not match the environment variables
of the current commit, then a hard restart is done instead of a soft
restart or replacement.
pmctl: Add env-get to get configured app env vars

Reads the environment variables stored in the strong-pm server. If
given, the pmctl client filters the list by name.
pmctl: Add env-unset to remove configured env vars

Internally add null as a special value for environment variables that
indicates deletion.

Expose this convention on the CLI as `sl-pmctl env-unset KEYS...`
Use released version of loopback-explorer

Closes #58
Use released version of loopback-explorer

Closes #58
1.3.1

 * Use released version of loopback-explorer (Ryan Graham)
Merge branch 'release/1.3.1' into production
Merge branch 'production'
install: Recursively set ownership of baseDir
internal: extract sl-pm-usage to usage file
install: add --set-env/-e for initial app env

Add the -e,--set-env option to sl-pm-install for setting initial env
vars for use by deployed applications. The environment specified by
this option is stored using the same internal store that the pmctl
get/set-env commands use.

The argument is parsed based on whitespace, including newlines, so it
is possible to do `slc pm-install --set-env "$(cat .env)"` to re-use
an existing .env file from a Heroku/12factor app.
internal: use persisted environment for --metrics

Move the STRONGLOOP_METRICS environment variable out of the Upstart job
and into the initial persisted environment.
docs: update command usage sections