Add an IncomingBox class that soledad client can use

Definition of done

a class ready to be used by leap mail, with proper unit tests

Discussion

Currently we're adding methods to the blobs as part of the incoming box specification. However, I'd prefer if we start adding methods to a single IncomingBox class - that will use delegation/proxy for most of the methods.

class IncomingBox(object):
   def __init__(self, namespace, blobmanager):
      self.ns = namespace
      self.blobmanager = blobmanager