Persisted environment variables for strong-pm hosted applications

 * [x] Create persisted environment on pm start
 * [x] Inject persisted environment into `prepare` and `run` phases
 * [x] Set environment variables via `pmctl env-set K=V [K2=V2...]`
 * [x] Get environment variables via `pmctl env-get [KEYS...]`
 * [x] Unset environment variables via `pmctl env-unset KEYS...`
 * [x] CLI argument parsing/validation
 * [x] Document CLI

Additional functionality for later PR:
 * Expose environment via REST API models as part of service state

Refs: strongloop-internal/scrum-nodeops#99
jshint abused me into not quoting dictionary keys when they don't have to be... and anyhow ls isn't quoted, so lets not quote this, either. Or quote them all, if you want.