Update for invalid input stream from IPN post #74

* Performs check to ensure the file_get_contents() function is not disabled and attempts to enable.
* Prior to PHP 5.6, php://input couldn't be reliably re-read.  This patch reads it once and works with that data via the raw_post_data variable.
* https://github.com/bitpay/magento-plugin/issues/74
Some web hosts disable this function.  You can't count on configurations to be the same across the board.