Use released version of loopback-explorer

Closes #58
install: Recursively set ownership of baseDir
internal: extract sl-pm-usage to usage file
Allow setting application env during installation

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.