add method to remove branch protection and doc example

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');
+    }
 }

test/Github/Tests/Api/Repository/ProtectionTest.php

@@ -39,6 +39,22 @@ public function shouldUpdateProtection()
         $this->assertEquals($expectedValue, $api->update('KnpLabs', 'php-github-api', 'master', $data));
     }
 
+    /**
+     * @test
+     */
+    public function shouldRemoveProtection()
+    {
+        $expectedValue = array('someOutput');
+
+        $api = $this->getApiMock();
+        $api->expects($this->once())
+            ->method('delete')
+            ->with('/repos/KnpLabs/php-github-api/branches/master/protection')
+            ->will($this->returnValue($expectedValue));
+
+        $this->assertEquals($expectedValue, $api->remove('KnpLabs', 'php-github-api', 'master'));
+    }
+
     /**
      * @return string
      */