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/Common/PPUserAgent.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace PayPal\Common;
+
+/**
+ * Class PPUserAgent
+ * PPUserAgent generates User Agent for curl requests
+ *
+ * @package PayPal\Common
+ */
+class PPUserAgent
+{
+
+    /**
+     * Returns the value of the User-Agent header
+     * Add environment values and php version numbers
+     *
+     * @param string $sdkName
+     * @param string $sdkVersion
+     * @return string
+     */
+    public static function getValue($sdkName, $sdkVersion)
+    {
+
+        $featureList = array(
+            'lang=PHP',
+            'v=' . PHP_VERSION,
+            'bit=' . self::_getPHPBit(),
+            'os=' . str_replace(' ', '_', php_uname('s') . ' ' . php_uname('r')),
+            'machine=' . php_uname('m')
+        );
+        if (defined('OPENSSL_VERSION_TEXT')) {
+            $opensslVersion = explode(' ', OPENSSL_VERSION_TEXT);
+            $featureList[] = 'openssl=' . $opensslVersion[1];
+        }
+        if (function_exists('curl_version')) {
+            $curlVersion = curl_version();
+            $featureList[] = 'curl=' . $curlVersion['version'];
+        }
+        return sprintf("PayPalSDK/%s %s (%s)", $sdkName, $sdkVersion, implode(';', $featureList));
+    }
+
+    /**
+     * Gets PHP Bit version
+     *
+     * @return int|string
+     */
+    private static function _getPHPBit()
+    {
+        switch (PHP_INT_SIZE) {
+            case 4:
+                return '32';
+            case 8:
+                return '64';
+            default:
+                return PHP_INT_SIZE;
+        }
+    }
+}