stdout is hardcoded in CarthageKit

[jspahrsummers]

Everything should be redirectable if we want CarthageKit to be useful in other applications.

[jspahrsummers]

Ideally, this would not be an “output stream” parameter passed to everything. Instead, we should yield enough information to callers (via returned signals) that they can log whatever they find interesting.

For example, buildScheme() could send an inner signal per scheme built, and subscribers could use the starting/stopping of those inner signals for logging events.