Removed Cart facade from coverage (does not provide a public inteterface)

Exclude migrations from coverage
Added tests for InstanceIdentifier
Added InstanceIdentifier support to store, restore and merge
Added weight to CanBeBought trait
Excluded CanBeBought trait from coverage temporarily

tests/Fixtures/Identifiable.php

@@ -0,0 +1,51 @@
+<?php
+
+namespace Gloudemans\Tests\Shoppingcart\Fixtures;
+
+use Gloudemans\Shoppingcart\Contracts\InstanceIdentifier;
+
+class Identifiable implements InstanceIdentifier
+{
+    /**
+     * @var int|string
+     */
+    private $identifier;
+
+    /**
+     * @var int
+     */
+    private $discountRate;
+
+    /**
+     * BuyableProduct constructor.
+     *
+     * @param int|string $id
+     * @param string     $name
+     * @param float      $price
+     */
+    public function __construct($identifier = 'identifier', $discountRate = 0)
+    {
+        $this->identifier = $identifier;
+        $this->discountRate = $discountRate;
+    }
+
+    /**
+     * Get the unique identifier to load the Cart from
+     *
+     * @return int|string
+     */
+    public function getInstanceIdentifier($options = null)
+    {
+        return $this->identifier;
+    }
+
+    /**
+     * Get the unique identifier to load the Cart from
+     *
+     * @return int|string
+     */
+    public function getInstanceGlobalDiscount($options = null)
+    {
+        return $this->discountRate;
+    }
+}