web3gw client for doing sell/buy to a liquidity pool for TFT/ETH

[robvanmieghem]

Needs V client + example scripts

• [x] https://github.com/threefoldtech/web3_proxy/issues/52

[robvanmieghem]

TODO:

• [x] Verification of https://github.com/threefoldtech/web3_proxy/issues/52
• [x] Add documentation to the manual
• [x] https://github.com/threefoldtech/web3_proxy/issues/112