use data-label attribute instead of data-type

Currently, a note is marked up like `<div class="note" data-type="aside">...` and this changes it to `<div class="note" data-label="aside">...`.

This allows the editor to edit cnx HTML (we don't use `data-type="note"` yet; everything's waiting on Aloha to change) and allows us to start using the same CSS when viewing the content online (see example below).

The next PR will change `class="note"` to `data-type="note"` and probably move `data-label="aside"` to `class="aside"` thus completing the transition to using `data-type` attributes.

## Example screenshots

(using the same CSS)

Online [link](http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@7.31:4#fs-id2834817):
![image](https://f.cloud.github.com/assets/253202/2199511/77c29b90-98d5-11e3-8189-6d3b5862c8f0.png)

In Aloha (using the same CSS):

![image](https://f.cloud.github.com/assets/253202/2199538/cb569df6-98d5-11e3-94e9-f72a6d084520.png)