use the same authentication method as the validators

[Ivshti]

solves #12

described here: https://github.com/AdExNetwork/aips/issues/32

Use the Relayer to verify EWT authentication tokens.

This will ensure we don't have to have blockchain-specific code in the Market, and we can use the same authentication method as the validator uses, so that tokens can be reused and the user will have to sign less messages on login