allow to sort by $meta-projection

Fix #607 exists() bug and assure compatibility with Laravel 5.1.19

Fix #607

Update QueryBuilder get method to run getFresh method

Following the latest update of the laravel framework (5.1.20), the BaseQuery builder that used to just run the getFresh method (overriden by the mongodb child builder) in the get function, now calls the runSelect method that relies on the mysql Grammar object being present that this builder does not need to run.

This update simply removes the parent get() call and directly calls it's own getFresh method, thus return the functionality to what it was before the laravel update.

Applied fixes from StyleCI

Stub Schema::hasColumn(), fixes #579

Allow EmbedsOneOrMany::saveMany to accept a collection

Fix Docblocks in Model class and Embed Relationship classes

DB Connection example updated to use env

Update installation instructions for L5 and Lumen