validations: support multi-key unique constraint

Modify the "unique" validator to accept additional property names to
narrow the space of rows searched for duplicates.

Example:

Consider `SiteUser` belongsTo `Site` via `siteId` foreign key.
Inside every site, the user email must be unique. It is allowed to
register the same email with multiple sites.

    SiteUser.validateUniquenessOf('email', { scopedTo: ['siteId'] });
Merge pull request #110 from strongloop/feature/multi-column-unique-validation

validations: support multi-key unique constraint