xctool should use the action-specific build configurations from the scheme.

Xcode's scheme editor lets you set a specific build configuration for
each of the actions (Run, Test, Profile, Analyze, Archive), but xctool
didn't know about these.  Instead it would default to 'Release' since
that's the default for command-line builds in most projects (from
http://d.pr/i/575l).

It seems like it would make more sense to just follow Xcode than
xcodebuild in this case.

By default, a scheme will specify Debug for the Run and Test actions, so
xctool's 'build' and 'test' actions will now default to the Debug
configuration.

Also by default, a scheme will specify Release for the Archive action,
so xctool's 'archive' action will default to the Release configuration.