Moved the Podio SDK callback injection to the request Future.

The old implementation took the callback implementations as
arguments to the `Provider` methods. These were unconditional
arguments, and the caller had to provide a value for them, even
so if just a null pointer.

The new implementation allows the caller to attach the callback
implementations of interest directly on the returned `Future`
from the `Provider` methods. This is a much more convenient way
of using a modern SDK and enables much more fluent and self
describing source code.
Switched to exception based error handling.

In this commit the `ErrorListener` interface has been removed.
The SDK now throws exceptions instead. A `PodioException` will
deliver any non-success status messages from the API.
Renamed the RequestFuture callback setter methods.
Removed merge/rebase artifacts.
Switched to Java Long based id notation (instead of Integer).
Fixed 'SimpleDateFormat' lint warning.
Added some missing attributes to space and org
Cleaned out unnecessary polymorphism in the SessionProvider.
Minor changes to member visibility in the Podio facade.
Fixed compiler warning.

Caused by a dangling unused import from a careless refactoring (shame on me).
Loosened member visibility in `VolleySessionClient`.
Narrowed down Podio facade member visibility.

The wider wisibility span turned out not to be needed in the end.
Added empty check methods for byte arrays.
Allowed `PodioException` to hold it's source JSON.
Removed unnecessary `PodioException` wrapping.

In some cases there was a risk for wrapping a PodioExeption with
another PodioException which doesn't make sense. These cases
have now been removed.
Prevented PodioExceptions from being wrapped by ExecutionException.

The RequestFuture is no longer wrapping PodioExceptions with
ExecutionExceptions. This allows direct access to the PodioException
and a much clearer view to the user on what was the reason for the
SDK failing in its operation.
Minor accessibility refactoring.
Enabled SSL socket factory injection into the Volley REST clients.
Enabled SSL socket factory injection through Podio facade.
Changed to match the changes in the SDK