Add curl as an optional method for communicating with PayPal server

I was getting a 403 forbidden error when using file_get_contents. curl worked fine for me so I added it as a parameter.

##### Summary of Changes
* Add paramters to plugin form to show options *file_get_contents* and *curl* (default file_get_contents for backwards compatibility)
* Update en-GB accordingly
* Add method getApiMethod() to get plugin parameters
* Add logic to switch between curl and file_get_contents