Improve project description

[cawel]

Current project description: "The Spreedly gem provides a convenient Ruby wrapper Spreedly's Subscriptions API."

First, I'd say there are a few words missing in that sentence. But more importantly, I believe it's no longer accurate. It took me a while to figure out if this project was solely focusing on the Subscriptions API. And after reading the history file [1] and the project code, I understand now that it's probably the best wrapper out there for the Spreedly API.

The project description should probably make it more obvious.

[1] https://github.com/spreedly/spreedly-gem/blob/master/HISTORY.md

[duff]

Just updated the description in Github. Thanks for noticing and mentioning that!