Introduce common provider core

[TheDanielDoyle]

The current implementation of the providers (DigitalOcean and GoDaddy) has a common structure. I believe it would cut down a lot of code to refactor the codebase to remove duplication where possible.

Examples of code that is almost identical and can be refactored out into a common core:

DigitalOceanAccountProcessor
DigitalOceanDomainProcessor
DigitalOceanDDNSService
GoDaddyAccountProcessor
GoDaddyDomainProcessor
GoDaddyDDNSService

• Refactor codebase
• Update documentation
• Update samples

[TheDanielDoyle]

Considering the amount of refactoring involved, it'd be high time to refactor over to a version 2.0 codebase.