Major code cleaning and refactoring of HttpCaller

Cleaned most of the codebase to follow modern code standards and made it
more consisten (e.g. private members all follow the same naming
standard).
Also removed the HttpCaller and replaced it with a nicer looking
PaysonClient that inherits from IPaysonClient.
Merge pull request #4 from UnitedNerds/master

Major code cleaning and refactoring of HttpCaller