Issue #12: GlobalStorageManager has now dummy impl

GlobalStorageManager now transfers files assuming that they all have
size of 1000. This is a nice step forward in implementation.
Issue #12 - DAGFile class, refactoring, file sizes

* Introduced DAGFile class and switched to it from pair of (string, size).
* Refactored some code: removed unnecesary one and simplified
  GlobalStorageManger
* Handled file sizes in GlobalStorageManager