support embedded usage

* only firefox and chrome are tested; IE is known not supported yet.
* `LER.addPopup` is prevented for embedded situation to prevent #16 .
* modify `Node#innerText = "text"` to
`Node#appendChild(document.createTextNode("text"))` to enable embedded
situation for Firefox.

Demo page is created on [Github
pages](http://kong0107.github.io/zhLawEasyRead); link of it is added in
`README.md`.