Channel payments contract

[mpapierski]

We forked the channels contract here: https://github.com/althea-mesh/simple-bidirectional-erc20-channel

After getting it to work althea-net/simple-bidirectional-erc20-channel#2 it should be integrated into the tests, with ganache.

Otherwise, we should mock out whole contract calls, and write the tests for the contract in the contract repo.

[mpapierski]

Its already agreed that we'll be fine testing the contract with Rust which will also verify that we know how construct function calls, and how to wait for events etc. So I'll close this one in favor of #21