Improve requesters API.

[khrynczenko]

Idea: Make possible to use different stations list providers and enable to change them at run-time.

By requester I mean object that that results in list of available stations. Result might come from request to some web-api or local database etc.
For now there is only one HTTP requester namely one that uses www.radio-browser.info web-api. Because of that there is no interface for general Requester which in result ties the program to use only this one particular wep-api.

Solution: Make Requesters into strategies. Factory for retrieving them.