GWT Editor support for Summernote and style fixes

Summernote has his own methods getCode and setCode to set field value, so it's not supported by GWT Editors. I have overriden getText and setText methods from base class ValueBoxBase and it works correctly with Editors now.

I have removed the summernote-bs3.min.css file, because it replaces to many original bootstrap styles and causes some troubles, for example cell table headers are rendered incorrectly. According to official Summernote installation guide [1], this file is not needed and only summernote.css should be included.

[1] http://hackerwins.github.io/summernote/features.html#installation-include