Add `Authereum` support

[auryn-macmillan]

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Description

Issue owner: raul@colony.io

We would like to support Authereum with the Purser library. For that a purser-authereum module should be created that allows for easy wallet access using purser.

This issue is considered done when:

• A purser-autherem module is created that is (presumably) using the Authereum [sdk],(https://docs.authereum.com/).
• The wallet implements the purser Common Wallet Interface
• A test suite is added for this module (see https://github.com/JoinColony/purser/tree/master/modules/tests).
• A video is provided presenting a login flow with purser using the purser-authereum module.

[KiChjang]

Applied on colony.io, now waiting for approval.

[auryn-macmillan]

Thanks @KiChjang!
No need to wait to be assigned the task on Colony, I'll do that and immediately finalize/payout the task when you make a successful PR.