[modify] change gulp task name

- gulp build -> gulp build:sources
- gulp espowerify -> gulp build:tests
[add] "gulp cover" task

Code coverage analysis task of gulp
[add] test setting files
[update] npm run scripts

- Remove:
  - npm run clean
  - npm run browserify
  - npm run compile

- Change:
  - npm run build
  - npm run test
[add] parameter callback() in "gulp test" task
[change] Specify AudioContext as an argument of Audio Class
[disable] A bug that does not play sound, Once remove a code
[update] Ano Gakki example files