modularize the codebase

* use separate modules
* write pure javascript
* do not use transpires
* do not use template engines

close #3
test on AppVeyor

ci.appveyor.com/project/ShinnosukeWatanabe/isogram
use gulp.js instead of Grunt

http://gulpjs.com/
report code coverage to coveralls.io

https://coveralls.io/r/shinnn/isogram
update readme

* use flat badges
* add more showcases
* add API docs

close #1