substitute unused variable name

This commit removes a ruby's warning.

* "assigned but unused variable"
declare an instance variable name in #initialize

This commit removes a ruby's warning.

* "instance variable @{ivar} not initialized"
remove circular loadings

This commit removes a ruby's warning.

* "loading in progress, circular require considered harmful"
Merge pull request #13 from kachick/improve-remove_warnings

Remove some warnings