Add ConfigureProxy and to use it to build configuration tree
 * receivers are required to call config_section recursively, instead of plugin instance
 * After configuration tree build process, root section node is combined to plugin instance
   * by setting root node parameters into instance variables

All existing tests have passed at this commit.
fix to show subsection stack in error message
separate ConfigureProxy to another file: configurable is too complex
fix to return self to convenience
add test-logger for specs
* Add config_argument to set section argument (<section ARG>) into specified param
* Write specs to configure nested subsections by config_section
move to spec/config
add specs for default behavior of ConfigureProxy, and its merging
add instance variable accessibilities check
add some method to operate (and seems not to overwrite configuration attributes)
add specs of Fluent::Config::Section
fix bug not to run correctly on 1.9.3/2.0.0
extract method definition
remove nonsense assigns
shrink dup code and add specs for twice-or-more default value specification
Merge pull request #298 from tagomoris/config_nested_sections

Configure nested sections