app: implement the main generator

Implement `yo loopback` as a thin wrapper around loopback-workspace's
`Project.createFromTemplate`. Allow the user to choose a custom project
name and select the project template he wants to use.
model: implement the Model generator

Implement `yo loopback:model` that calls Project methods to update
the project files and define a new Model.
datasource: implement the DataSource generator

Implement `yo loopback:datasource` that calls Project methods to update
the project files and define a new DataSource.
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.