relayer/dapp backend

[Ivshti]

separate component (relate dto #10) that acts as the dapp backend and relays transactions is needed

it's specific to the eth implementation of AdEx

full list of what it can do:

• counterfactually deploy identity contracts
• relay transactions to existing identity contracts
• using deploying counterfactually, it should also be able to create an identity funded by an AdEx grant

[Ivshti]

a spec is now written, will be moved here at some point