Pass response type when calling format_response(). fixes #8

*Class now takes a `$config` array upon instantiation for sensitive P…

…ayPal API information.

* Added a `clear_items()` function to clear items added when doing multiple transactions one after another.
* Updated to use PayPal API version 53.0 (was using 3.0)
* Added `manage_recurring_payments_profile_status()` function.
* Changed various function and variable names to be consistent across the class.
* Lots of smaller changes, see commit messages for full details.

Merge remote branch 'origin/master'

Fixed spelling mistake.

Version 0.7

Version 0.6

Version 0.5.2