Refactor and support v1.0-ropsten.8

[katat]

Refactor based on the data structure changes in https://github.com/hubiinetwork/nahmii-contracts/releases/tag/v1.0-ropsten.6

• [x] Support Null Settlement
• [x] Refactor getCurrentPaymentChallenge into different functions for checking the challenge statuses. These functions will wrap the contract functions of proposalNonce, proposalExpirationTime, hasProposalExpired, proposalStageAmount, proposalStatus
• [x] Refactor depositTokens to use receiveTokens which has an added argument balanceType
• [x] Update abis

[liamaharon]

@katat should we update this issue to support v1.0-ropsten.7 instead?

[katat]

@liamaharon Yes, I will continue the refactor on the branch https://github.com/hubiinetwork/nahmii-sdk/pull/40