Upgrade contracts to GSN network

[riusricardo]

From a GSN network team member.

This is currently the most active GSN app on xdai, with 50-100 hits every month.
(BTW: nothing of this is "private knowledge". its simply from parsing the GSN events on the blockchain)

Also, I want to let you know we're retiring the old 0.3.1 version, and soon there will be no relays, so its better to upgrade your code to the latest "tabookey-gasless" version, and use the production relayhub.
the same relayhub address is published on all networks (xdai, mainnet, rinkeby, ropsten, kovan..)

It is a good time to upgrade contracts and add Token compatibility to the burner wallet. I'll close the PR #184 to create a better option and use OpenZeppelin's GSN code.
cc: @austintgriffith

[austintgriffith]

Fantastic. Let me know how I can help.

[6proof]

looking forward to testing and contributing!