Added spot price functionality and clarified price descriptions in README

Added a `spot_price` method to enable an easy GET request to '/api/v1/prices/spot_rate.' Mirroring the API, the `spot_price` method cannot take a `quantity` argument.

I also added some minor clarifications and examples to the **_Check bitcoin prices_** section of the README. The existing description for the `sell_price` method shows a `buy_price` example.