Added isSubscribed method (#96)

* Added isSubscribed method

Takes a users email, uses the getMember method, and checks if firstly
the user exists in the list, and then if that users status is subscribed

* Code Cleanup

* Updated README.md to incorporate changes

* Small change on README

README.md

@@ -36,6 +36,9 @@ Newsletter::getMemberActivity('lord.vetinari@discworld.com');
 //Get the members for a given list, optionally filtered by passing a second array of parameters
 Newsletter::getMembers();
 
+//Check if a member is subscribed to a list
+Newsletter::isSubscribed('rincewind@discworld.com');
+
 //Returns a boolean
 Newsletter::hasMember('greebo@discworld.com');
 
@@ -210,6 +213,12 @@ There's also a convenience method to check if someone is already subscribed:
 Newsletter::hasMember('nanny.ogg@discworld.com'); //returns a bool
 ```
 
+In addition to this you can also check if a user is subscribed to your list:
+
+```php
+Newsletter::isSubscribed('lord.vetinari@discworld.com'); //returns a bool
+```
+
 ### Creating a campaign
 
 This is how you create a campaign:

src/Newsletter.php

@@ -137,6 +137,27 @@ public function hasMember($email, $listName = '')
         return true;
     }
 
+    /**
+     * @param string $email
+     * @param string $listName
+     *
+     * @return bool
+     */
+    public function isSubscribed($email, $listName = '')
+    {
+        $response = $this->getMember($email, $listName);
+
+        if (! isset($response)) {
+            return false;
+        }
+
+        if ($response['status'] != 'subscribed') {
+            return false;
+        }
+
+        return true;
+    }
+
     /**
      * @param $email
      * @param string $listName