We will convert test/ to TypeScript.
src/ is alredly TypeScripted #68

Porposes

• Improve coverage
  • I found that some wrong type on source code fix(TypeScript): fix Store#onChange return type #143
  • It improve the coverage
• Re-Enable power-assert Re-enable power-assert #83
  • espower-typescript help to use power-assert
• Reduce compile/build time
  • Remove babel and use only tsc
  • Improve build time I think.
• Refactoring tests
  • For example, Store is an abstract class, But test case directly new Store.
  • We should refactor test cases.