Update exists? to return true if group exists

.gitignore

@@ -6,3 +6,6 @@ pkg/*
 # YARD
 .yardoc
 yardoc/
+
+test/example*
+.ruby-*

lib/simple_config.rb

@@ -102,7 +102,7 @@ def unset(key)
     # @param  [Symbol] The key to check.
     # @return [Boolean] True if the key is set.
     def exists?(key)
-      @settings.key?(key)
+      @settings.key?(key) || @groups.key?(key)
     end
 
     def set?(key)

test/simple_config_test.rb

@@ -57,6 +57,11 @@ def test_exists_should_return_whether_variable_isset
     assert(@config.exists?(:foo))
     @config.unset(:foo)
     assert(!@config.exists?(:foo))
+    @config.group(:bats) do
+      set :foo, 'bar'
+    end
+    assert(@config.exists?(:bats))
+    assert(@config.bats.exists?(:foo))
   end
 
   def test_exists_should_consider_empty_values_as_set
@@ -103,7 +108,7 @@ def test_should_load_and_parse_external_config_as_ruby_in_context_of_config_inst
     @config.load('external_config.rb')
   end
 
-  def test_should_laod_and_parse_external_config_as_yaml_in_context_of_config_instance
+  def test_should_load_and_parse_external_config_as_yaml_in_context_of_config_instance
     parser = stub('YAMLParser')
     SimpleConfig::YAMLParser.stubs(:parse_contents_of_file).with('external_config.yml').returns(parser)
     parser.expects(:parse_into).with(@config)