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.

composer.json

@@ -14,7 +14,8 @@
     "require": {
         "php": ">=5.3.0",
         "ext-curl": "*",
-        "ext-json": "*"
+        "ext-json": "*",
+        "psr/log": "1.0.0"
     },
     "require-dev": {
         "phpunit/phpunit": "3.7.*"