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

README.md

@@ -128,5 +128,5 @@ There are two kinds of tests that we include in our sdk package. Unit tests, and
 
    * [API Reference](https://developer.paypal.com/webapps/developer/docs/api/)
    * [Reporting issues / feature requests] (https://github.com/paypal/rest-api-sdk-php/issues)
-   
+   * [Pizza App Using Paypal REST API] (https://github.com/paypal/rest-api-sample-app-php)
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