First pass on implementing PSR logger interface

- Created a separate PayPalLogger as default implementation.
- Enabled `log.Adapter` as a configuration to pass name of custom
  logger.

lib/PayPal/Log/PayPalLogger.php

@@ -0,0 +1,92 @@
+<?php
+
+namespace PayPal\Log;
+
+use PayPal\Core\PayPalConfigManager;
+use PayPal\Core\PayPalLoggingManager;
+use Psr\Log\LoggerInterface;
+
+
+class PayPalLogger implements LoggerInterface {
+
+    /**
+     * Default Logging Level
+     */
+    const DEFAULT_LOGGING_LEVEL = 0;
+
+    /**
+     * Configured Logging Level
+     *
+     * @var int|mixed
+     */
+    private $loggingLevel;
+
+    /**
+     * Configured Logging File
+     *
+     * @var string
+     */
+    private $loggerFile;
+
+    public function initialize() {
+            $config = PayPalConfigManager::getInstance()->getConfigHashmap();
+        if (!empty($config)) {
+            $this->loggerFile = ($config['log.FileName']) ? $config['log.FileName'] : ini_get('error_log');
+            $loggingLevel = strtoupper($config['log.LogLevel']);
+            $this->loggingLevel =
+                (isset($loggingLevel) && defined(__NAMESPACE__ . "\\PayPalLoggingLevel::$loggingLevel")) ?
+                    constant(__NAMESPACE__ . "\\PayPalLoggingLevel::$loggingLevel") :
+                    PayPalLoggingManager::DEFAULT_LOGGING_LEVEL;
+        }
+    }
+
+    public function emergency($message, array $context = array())
+    {
+        $this->error($message, $context);
+    }
+
+    public function alert($message, array $context = array())
+    {
+        $this->error($message, $context);
+    }
+
+    public function critical($message, array $context = array())
+    {
+        $this->error($message, $context);
+    }
+
+    public function error($message, array $context = array())
+    {
+        $this->initialize();
+        error_log("[" . date('d-m-Y h:i:s') . "] ERROR: $message\n", 3, $this->loggerFile);
+    }
+
+    public function warning($message, array $context = array())
+    {
+        $this->initialize();
+        error_log("[" . date('d-m-Y h:i:s') . "] WARNING: $message\n", 3, $this->loggerFile);
+    }
+
+    public function notice($message, array $context = array())
+    {
+        $this->initialize();
+        error_log("[" . date('d-m-Y h:i:s') . "] NOTICE: $message\n", 3, $this->loggerFile);
+    }
+
+    public function info($message, array $context = array())
+    {
+        $this->initialize();
+        error_log("[" . date('d-m-Y h:i:s') . "] INFO: $message\n", 3, $this->loggerFile);
+    }
+
+    public function debug($message, array $context = array())
+    {
+        $this->initialize();
+        error_log("[" . date('d-m-Y h:i:s') . "] DEBUG: $message\n", 3, $this->loggerFile);
+    }
+
+    public function log($level, $message, array $context = array())
+    {
+        $this->debug($message, $context);
+    }
+}