add getBalance method

src/Client.php

@@ -127,6 +127,30 @@ class Client
         return null;
     }
 
+    public function getBalance(): ?float
+    {
+        $this->_id++;
+
+        $this->_prepare(
+            '',
+            'POST',
+            [
+                'method' => 'getbalance',
+                'params' => [],
+                'id'     => $this->_id
+            ]
+        );
+
+        $response = $this->_execute();
+
+        if (isset($response['result']) && is_float($response['result']))
+        {
+            return $response['result'];
+        }
+
+        return null;
+    }
+
     public function getBlockHash(
         int $block
     ): ?string