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

composer.json

@@ -14,8 +14,7 @@
     "require": {
         "php": ">=5.3.0",
         "ext-curl": "*",
-        "ext-json": "*",
-        "paypal/sdk-core-php": "2.5.*"
+        "ext-json": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "3.7.*"