Add support for Kleros Arb Provider

[dan13ram]

• Check arbitration costs to create a dispute #28
• Appeal a ruling (contract) #29
• Submit evidence (contract) #30
• Submit evidence (UI) #31
• Appeal a ruling (UI) #32

[plor]

I think it would make sense to separate the kleros arbitration from the escrow release so it can go through the entire lifecycle of the kleros court and then that can be used to release funds to the appropriate parties (client, provider, arbitrator).