According to HelloJs doc, async methods return Promise A+ compliant thenable. So added interface for thenable and changed return type of login, logout and api methods.
This update will also give you the ability to use import/require imports in your projects like this:
`import * as hello from 'hellojs';`
or
`import hello = require('hellojs');`
Otherwise, TS will always say module 'hellojs' is undefined (if included as `import 'hellojs`)
