feat(ModelBuilder): adds the setUrlPrefix builder method.

Closes #34
refactor($restmod): $urlFor functions now receive item's $pk (instead of item)

This is to remove duplicate responsabilities.
feat(ModelBuilder): adds setPrimaryKey function.

The setPrimaryKey function allows the default primary key to be changed.

Breaks $build, it no longer allows a private key to be passed directly,
for that use $new or $build({ id: X })
bumps v0.12.0