Add `Portis` support

[auryn-macmillan]

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Description Issue owner: [email protected]

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

This issue is considered done when:

• A purser-portis module is created that is (presumably) using the Portis [sdk],(https://docs.portis.io/#/).
• 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-portis module.