WIP: rework response handling

Added a http_utils class to channel the response handling and so far converted the info, balances and api class to use this new structure.

In the mean time I'm also cleaning up code wherever I see fit and moving forward to a structure where we can separate the express/http logic from the api logic that talks with ripple-lib.

Primarily looking for input on my direction.

As of now, previous behavior is still intact, just the internal structure has changed slightly. So feel free to merge if we agree on this direction.