Jp add postinstall

As discussed with @rrees this feature will copy the default `bin/run-tests.sh` into the root of a given project.

This seems like a nice way of adding boilerplate to existing scribe-plugins. Not sure how far you would like to take this @rrees @theefer ?

Ideally this will just copy the script into your plugin directory. In your plugin you should wrap this with npm and provide the env there.