Implement project channel management

[Suremaker]

With #2 implemented it is possible to download and upload project variables that are channel specific.
The project channels are not managed by OctopusProjectBuilder however.

This ticket should add support for managing project channels.

[Suremaker]

Update FakeChannelRepository.cs in tests projects to properly manage channels in tests.
Refactor VariableConverter to decrease number of parameters.

[matthewmarchus]

This is resolved in #25 for your review!