Rename arbiter property

[lightclient]

I think it would be best to rename the arbiter field on the bounty so it isn't confused with a Delphi arbiter. Additionally, I think if we're not going to make this field an array of address we should remove it. It's weird to have a +1 on the bounty that can only accept fulfillments. It should be expandable if anything.

[villanuevawill]

@c-o-l-o-r specifically you are saying we should make the arbiter an array of addresses right?

[mbeylin]

I've renamed them to approvers now and turned it into an array, where the issuer can remove people from the array, or push onto it with ease.

[lightclient]

@mbeylin amazing!