Fix label covering issue

I just found simple solution of Issue #68 

add `display: inline-block;` code in note-editor class.

I used django-summernote.css file and added it to widgets.py file.

Following is fixed screenshot.

![label-covered_fixed](https://cloud.githubusercontent.com/assets/3654082/4946501/08e44f44-6615-11e4-9cd6-7d111f82c6bc.png)

I hope it helps.