add Coveralls

* CoverallsでRailsアプリのテストのカバレッジ率を取得する手順メモ: http://qiita.com/knt45/items/58cacf895c0e5ad9228b
Merge branch 'travis-ci' into coveralls
resolve conflict of README.md
Merge branch 'MiCHiLU-coveralls'