Adds support to set the collection name

This merge adds the support to the third argument of jugglingdb's "describe()", that can be used to configure the table name (in this case, collection name).

Now one who creates a Model with:
User = define('User', {
  name: {type: String}
}, {
  table: 'system_users'
});

Jugglingdb will use the collection "system_users" instead of "User".

But if the user don't give the 3rd argument, it will use the "User" collection.