Define system requirements (bcmath or gmp PHP extention)

Fixes #5

Plugin.php

@@ -139,6 +139,21 @@ class Plugin extends \Mibew\Plugin\AbstractPlugin implements \Mibew\Plugin\Plugi
         return array();
     }
 
+    /**
+     * Returns plugin's system requirements
+     *
+     * @return type
+     */
+    public static function getSystemRequirements()
+    {
+        if (extension_loaded('gmp')) {
+            return array('ext-gmp' => '*');
+        }
+        else {
+            return array('ext-bcmath' => '*');
+        }
+    }
+
     /**
      * Format locale name using "xx-XX" format.
      *

README.md

@@ -2,6 +2,7 @@
 
 Provides Geo IP information for other plugins.
 
+*Requires either [GMP (GNU Multiple Precision)](http://php.net/manual/en/book.gmp.php) or [BC Math (BCMath Arbitrary Precision Mathematics)](http://php.net/manual/en/book.bc.php) PHP extension.*
 
 ## Installation