lib: fetch pricing from API

[aufziehvogel]

Pricing is a bit different than the other resource types. It only provides a single API endpoint /pricing. On this it only returns a single dictionary. All other resource types return an array of resources and have a /<resource>/<id> endpoint for single resources.

Our concept of <X> and <X>Resource classes currently maps to the latter concept.

I tried two different approaches for a user of our client:

Stick to the same concept as closely as possible. This is the one implemented now.

pricing = client.pricings.fetch
expect(pricing.currency).to be_a String

Only use a Pricing class or forward all calls from PricingResource to Pricing.

expect(client.pricing.currency).to be_a String

The second approach in my opinion would be nicer for a user, but had some complications. Without special additions to the code the client will on each call to client.pricing issue a new API request. This would cause needlessly many API requests if a user checks a few prices. A fix to this would be that the Client has to cache some information (pricing or pricing resource). Currently, it just creates a new instance of <X>Resource whenever an attribute is called.

Closes #28