This commit introduces a first version of a `VaultFactory` that later
will be extended to be capable of instantiating reward vaults and
possible keep track of vault instances per owner.
As a first step, this implementation comes with a `createVault()`
function which takes care of creating vaults.
Because `VaultFactory` also knows about `StakeManager` it can derive the
manager's address and stake token from it when creating vaults, allowing
the API to be without arguments.
Partially addresses #37
r4bbit