Add rake task to check cronotab.rb syntax

spec/assets/bad_cronotab.rb

@@ -0,0 +1,12 @@
+# This is an example of a bad cronotab for tests
+
+class TestJob
+  def perform
+    puts 'Test!'
+  end
+end
+
+# This is an error, because you can use `on` options with
+# a period less than 7 days.
+
+Crono.perform(TestJob).every 5.days, on: :sunday

spec/assets/good_cronotab.rb

@@ -0,0 +1,9 @@
+# This is an example of a good cronotab for tests
+
+class TestJob
+  def perform
+    puts 'Test!'
+  end
+end
+
+Crono.perform(TestJob).every 5.seconds