Add Hybrid `has` and `whereHas` functionality

Update README.md

Update more guide for $slide and $skip

Change
DB::collection('items')->project(['tags' => array('$slice' => 1]))->get();
To
DB::collection('items')->project(['tags' => array('$slice' => 1)])->get();

[Bug Fix][Validation][Security] Fix working of unique and exists

Aggregation shouldn't mutate querybuilder

Add default attribute to `EmbedsOneOrMany::create` method

Tiny wording change, change "table" to "collection"

Fixes missing QueryGrammar in Connection class

Support Auto-Discovery

Fix: Missing return statement

I think this meant to be "collection" not "table".

This brings it inline with the base Eloquent `create` method, and allows us to call this method without providing any attributes.

Add direct geospatial index support

Since #371 was raised back in 2014 I’ve wanted a way to improve and simplify adding geospatial indexes.

`with` tests for hybrid relations and dont constrain mysql->mysql relations but we do need to constrain the mongo ones

Aggregations (like $query->count() ) modifies the internal state of the queryBuilder object, therefore the $query looses eg. the projection.

Eloquent solves this in the following way:
https://github.com/laravel/framework/blob/5.3/src/Illuminate/Database/Query/Builder.php#L2036

Fix #1157 (Model::has() - ErrorException: Array to string conversion)

With ‘BelongsToMany’ relationships calling the function `Jenssegers\Mongodb\Eloquent\Model::has` can throw an ErrorException with ‘Array to string conversion’.

By flattening the collection here we can avoid triggering the exception.

Issue #1131 fix

- fixed possiblilty to specify index names when creating them
- also fixed docstring comments so code completion works
  properly when using IDEs

* fix query builder delete

* tweak a bit

* tweak the test