Extract common and specific parts for provider

[jaapterwoerds]

While working on this PR(#16), @jrdi suggested to first do some refactoring on the provider to separate the generic and specific parts for a provider.

The simplest structure I can think of is a base class provider which takes the current provider code, wraps it in a class but leaves one method unimplemented(namely the one method which does a call to the data provider get data for one ticker for a given period).