Allow to enable logging for debugging

[real34]

The Elasticsearch client cannot be initialized with logging enabled yet. This issue is about allowing to do so for debugging purposes.

See #2 (comment)

[real34]

Quick and dirty tip to achieve this temporarily in dev:

diff --git a/src/lib/vendor/psr/log/Psr/Log/NullLogger.php b/src/lib/vendor/psr/log/Psr/Log/NullLogger.php
index 553a3c5..8a8c225 100644
--- a/src/lib/vendor/psr/log/Psr/Log/NullLogger.php
+++ b/src/lib/vendor/psr/log/Psr/Log/NullLogger.php
@@ -22,6 +22,7 @@ class NullLogger extends AbstractLogger
      */
     public function log($level, $message, array $context = array())
     {
+        echo $message;
         // noop
     }
 }