upload with postage stamps: stamper

[acud]

As per the architectural proposal that deals with uploading with postage stamps, an initial mock Stamper implementation is needed as follows:

• Stamper interface definition as seen below
• The returned postage stamp signature should be a signature as follows: sign(sha3(chunk address(32bytes)+batch id(32byte),key)

type Stamper interface {
    Stamp(chunk) postage.Stamp
}

postage service and stampers implementation follows
Architectural proposal: stamping chunks during upload

related to #654