Add curl as an optional method for communicating with PayPal server
* Add paramters to plugin form to show options (default
file_get_contents)
* Update en-GB accordingly
* Add method getApiMethod() to get plugin parameters
* Add logic to switch between curl and file_get_contents
Fix missing ending blank line.
Merge pull request #29 from plutz/development

Add curl as an optional method for communicating with PayPal server