Created general class for easy dynamic bindings to API.

Hi Teddy!

The API class is capable to perform requests according to API (https://www.aftership.com/docs/api/3.0). Tested on basic GET requests so far.

Please note, module does not contain bindings to specific API methods (e.g. /couriers, /trackings, etc.), but rather create specific call based on a call string (see README example to get the general idea).

My short plan: test on POST/PUT/DELETE, rate limits, exceptions on errors, APIv3 class (specific simplifications based on API conventions), packaging and publishing.