Defined AbstractStorageProvider and implemented JsonStorageProvider.

Server Class is now dependant on AbstractStorageClass and expects derivate of such class as a first argument allowing creation of different dns storage classes. Derivate of AbstractStorageProvider requires implemented get_answer method which handles query. As an example json storage provider is implemented at the moment. Feature tested on linux.
Merge pull request #2 from ivanstan/master

Defined StorageProvider