relayTokens support in scenarios where ERC677 is used

[akolotov]

As part of #295 two ways to transfer tokens with specifying an alternative receiver were implemented:

1. for scenarios where generic ERC20 tokes are involved it is necessary to use sequential pair of approve and relayTokens methods
2. for scenarios where ERC677 compatible tokens are used it is enough to call transferAndCall

Even if the second way is simpler from a Dapp point of view and does not require two transactions, Dapp still needs to support the first way since generic ERC20 tokens are more widespread. So, in order to operate with both type of tokens the Dapp's developers should maintain two different pieces of code to achieve similar functionality. As a proof that consistency between operations with two different tokens should be kept the following fact can be used: in the past there was a special activity to allow ERC677 tokens used by the bridge behave similarly to generic ERC20 - a user could transfer tokens through the bridge both with transferAndCall and transfer.

That's why it seems logical to support approve-and-relayTokens approach for the bridges where ERC677 tokens are involved together with transferAndCall approach for the scenarios with the alternative receiver.

Additional discussion can be found here.