refs #10 just Exec rails g model ExpenseCategory

> rails g model ExpenseCategory name:string color:string
refs #10 Implement ExpenseCategory model
refs #10 just Exec rails g scaffold_category ExpenseCategory
refs #10 Remove unnecessary files created by generator
refs #10 Delete unnecessary json code
refs #10 Delete unnecessary comments
refs #10 Implement minimum codes just for pass the specs
refs #10 Refactor expense_category specs

* Use let
* Use sequenced name for factory
refs #10 Implement views
refs #10 Remove validation of uniquness for category name
refs #10 Use null false for migration of ExpenseCategories
refs #10 Remove unnecessary specs

because uniqueness validation was deprecated
refs #10 Use random selected color for factory of expense_category
Merge pull request #15 from upinetree/10_crud_expense_cateogry

10 crud expense cateogry