Refund functionality and Issue fixes

- Implemented Refund functionality
- Fixed issue https://github.com/bitpay/php-bitpay-client/issues/128
- Added ability to retrieve the pairing code when creating a merchant pairing token
it is correctly {@inheritdoc}

https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Form/Form.php#L185

and http://manual.phpdoc.org/HTMLSmartyConverter/HandS/phpDocumentor/tutorial_tags.inlineinheritdoc.pkg.html