Provide an AbstractApiProvider to clean up paths

[JasonTheAdams]

There are many places where a provider/model sets the various API endpoint paths. Each places concatenates the path and uses things like ltrim(). It's redundant and opens up for simple mistakes. It would be great to have an AbstractApiProvider class (and likely a corresponding provider interface) that is aware of a base URL and has an AbstractApiProvider::url($path) counterpart, which cleans this up.

[felixarntz]

@JasonTheAdams Let's implement this this week, ideally to release in 0.2.0. I'd love to do a fast follow with some crucial enhancements and fixes, maybe we can already release end of this week or beginning of next week.