Use Logger for non-result output

This adds the Logger class for log output, all of which goes to STDERR,
and uses puts to send only the command result to STDOUT.

lib/vmfloaty/logger.rb

@@ -0,0 +1,11 @@
+require 'logger'
+
+class FloatyLogger < ::Logger
+  def initialize
+    super(STDERR)
+    self.level = ::Logger::INFO
+    self.formatter = proc do |severity, datetime, progname, msg|
+        "#{msg}\n"
+    end
+  end
+end