Adds new fastly cache backend

lib/cartodb/cache/backend/fastly.js

@@ -0,0 +1,18 @@
+var FastlyPurge = require('fastly-purge');
+
+function FastlyCacheBackend(apiKey, serviceId, softPurge) {
+    this.serviceId = serviceId;
+    this.fastlyPurge = new FastlyPurge(apiKey, { softPurge: softPurge || true });
+}
+
+module.exports = FastlyCacheBackend;
+
+/**
+ * @param cacheObject should respond to `key() -> String` method
+ * @param {Function} callback
+ */
+FastlyCacheBackend.prototype.invalidate = function(cacheObject, callback) {
+    this.fastlyPurge.key(this.serviceId, cacheObject.key(), callback);
+};
+
+module.exports = FastlyCacheBackend;