Add http ['cert'] configuration option

Merge pull request #56 from srchase/http-cert-config

Author: SCalwas

doc_source/guide_configuration.rst

@@ -455,6 +455,41 @@ created by the SDK.
 
 The SDK supports the following configuration options:
 
+.. _http_cert:
+
+cert
+----
+
+:Type: ``string|array``
+
+Specify the PEM formatted client side certificate.
+
+* Set as a string for the path to only the certificate file.
+
+.. code-block:: php
+
+    use Aws\S3\S3Client;
+
+    $client = new S3Client([
+        'region'  => 'us-west-2',
+        'version' => 'latest',
+        'http'    => ['cert' => '/path/to/cert.pem']
+    ]);
+
+* Set as an array containing the path and password.
+
+.. code-block:: php
+
+    use Aws\S3\S3Client;
+
+    $client = new S3Client([
+        'region'  => 'us-west-2',
+        'version' => 'latest',
+        'http'    => [
+            'cert' => ['/path/to/cert.pem', 'password']
+        ]
+    ]);
+
 .. _http_connect_timeout:
 
 connect_timeout