General refactor of package release
General refactor of package release

- Releaser is used by other packages, so it becomes an exported type.
- Use consistent (r *Releaser) method receiver instead of (s *Releaser).
- Use consistent in/out logging for each Release* method.
- releaseActionNop was always invoked with fmt.Sprintf, so save some
  callsite noise by having it take format and args params directly.
- just{Result,Error} helper methods added indirection and cognitive
  overhead but didn't reduce SLoC or do much helping. Removed.

squaremo/flux,weaveworks/flux,squaremo/flux,weaveworks/flux