code style cleanup

Add `jshint .` as a pretest script.

Modify the jshint config - remove "white" (Crockford's style),
add "maxlen".

Fix issues found by jshint plus few other.
acl: implement the DataSource generator

Implement `yo loopback:acl` that calls Project methods to update
the project files and add a new ACL entry for a given model.