[RFR]: Added Origin CA Certificate endpoint (#123)

* Added Certificate endpoint

* Implement all calls in /certificates endpoint

* Added to travis more php versions

* Fix package compatibility

* Added CertificateTest

* Added test for Certificate endpoint

* Added more tests

* Updated README

Co-authored-by: haphan <thanhha.phan@myrepublic.net>

tests/Fixtures/Endpoints/listCertificates.json

@@ -0,0 +1,19 @@
+{
+  "success": true,
+  "errors": [],
+  "messages": [],
+  "result": [
+    {
+      "id": "328578533902268680212849205732770752308931942346",
+      "certificate": "some-cert-data",
+      "hostnames": [
+        "example.com",
+        "*.example.com"
+      ],
+      "expires_on": "2014-01-01T05:20:00.12345Z",
+      "request_type": "origin-rsa",
+      "requested_validity": 5475,
+      "csr": "some-csr-data"
+    }
+  ]
+}