Add Nano support for Bountysource

Nano (formerly known as Raiblocks) is a cryptocurrency that focus on fast, feeless payments in a network that can support high transaction volumes. This makes it very suitable for small payments, which I think should make it interesting for a service like Bountysource.

You can find more information on the web page: https://nano.org

No files, tests, or entry points are named in the issue. Start by surveying the existing payment integrations and Bountysource payment flow, then identify the requirements and tests needed for Nano support; the work is done when the integration is implemented and verified against those requirements.

Written by the indexing model from the issue text.