example: use `npm pretest` from workspace

loopback-workspace provides `pretest` script and JSHint config
out of the box, we don't need to setup it ourselves.
example: fix a bug introduced by yeoman upgrade

yeoman-generator 0.17 changed the way how `copy` operation works, which
broke `yo loopback:example`.

This commit changes the `copyFiles` action to explicitly force yeoman
to execute all copy operations.
example: add LICENSE, README and create-load.js