add method to remove branch protection and doc example

Author: bobeagan

doc/repo/protection.md

@@ -33,3 +33,11 @@ $params = [
 ];
 $protection = $client->api('repo')->protection()->show('twbs', 'bootstrap', 'master', $params);
 ```
+
+### Remove branch protection
+
+> Requires [authentication](../security.md).
+
+```php
+$protection = $client->api('repo')->protection()->remove('twbs', 'bootstrap', 'master');
+```

lib/Github/Api/Repository/Protection.php

@@ -52,4 +52,18 @@ public function update($username, $repository, $branch, array $params = array())
     {
         return $this->put('/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/branches/'.rawurlencode($branch).'/protection', $params);
     }
+
+    /**
+     * Remove the repo's branch protection
+     *
+     * @link https://developer.github.com/v3/repos/branches/#remove-branch-protection
+     *
+     * @param  string $username   The user who owns the repository
+     * @param  string $repository The name of the repo
+     * @param  string $branch     The name of the branch
+     */
+    public function remove($username, $repository, $branch)
+    {
+        return $this->delete('/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/branches/'.rawurlencode($branch).'/protection');
+    }
 }