Merge branch 'master' into status-formatter-tabular

- Migrated FormatTabular to status_formatters.go.
- Migrated return error to github.com/juju/errors
- Removed debugging/panic in an impossible situation.
- Migrated towards use of stack-local byte-buffer rather than a heap-based pointer dereference.

Conflicts:
	cmd/juju/status.go
	cmd/juju/status_test.go