SampleOffer: allow changing the contractor

[chfast]

It is just an idea.

How about allowing the Client and the Contractor changing the address of the Contractor. That can be useful in something happens to the old account, the Contractor wants to upgrade, the DAO wants to change the contractor.

[LefterisJP]

It could be considered in later iterations of the contract. For now we are very close to a code freeze for the first version we want to deploy.

With this version, the idea is that if at any point you would like to renegotiate something or change contractor, you "fire" the contractor and renegotiate with a completely new contract.