Removing Dependency from SDK Core Project

- Copied files required for Rest API SDK
- Removed PPApiContext and directly connected APIContext with PPConfigManager
- Removed duplicate data storage of configuration and credentials.
- Code Style Fixes
- Remove build.xml file as it is not required anymore
- Updated the samples
- Updated the documentations

lib/PayPal/Core/PPLoggingLevel.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace PayPal\Core;
+
+/**
+ * Logging Levels.
+ * Class containing all the constants for Logging Levels.
+ */
+class PPLoggingLevel
+{
+
+    // FINE Logging Level
+    const FINE = 3;
+
+    // INFO Logging Level
+    const INFO = 2;
+
+    // WARN Logging Level
+    const WARN = 1;
+
+    // ERROR Logging Level
+    const ERROR = 0;
+}