Add core Rehive services

[ConnorMac]

This is not a high priority issue.

Currently only the core endpoints are supported and it would be great to have all the services endpoints added to the SDK. To prevent the main Rehive object from getting a little too polluted I think all services should be extra imports.

• Investigate a nice import system for services which can share a client with the main Rehive object
• Add Bitcoin service
• Add Ethereum Service
• Add Stellar service
• Add Notification service
• Add Conversion service

[OdinsPlasmaRifle]

Do we want this as part of the SDK? Or as individual SDKs rather?

Also, ICO service has been deprecated. Newest core project is the Conversion service.

[ConnorMac]

Probably best to manage these separately. Although for this lesser ones I think looking into some kind of auto generator based on the API would be cool since it could become tiresome manually dealing with them all.