Add Bare Option for Tablesaw CSS

This is in response to my issue #57 request. This version of a solution for that request includes the following things.

## Currently Included Changes

* I added additional build steps to create a version of the tablesaw.css file *without* the tablesaw.skin.css source file, called "tablesaw.bare.css"
* I added an additional html test file to ensure that existing javascript & tests work with the bare file as they do with the standard file
* I added a specific "bare.html" demo file using this css file and linked it to the rest of the demo files
* I added a brief section in the readmeexplaining the usage of the css file

## Potential Issues

*  I didn't add any specific new tests besides just ensuring that the existing quite worked with the new css, as there are no actual javascript code changes
* Removing the skin css file does not remove *all* style information. There are still some specified drop shadows, paddings/heights, button styles, etc. It's possible that this mode would be better served by additional css modifications beyond  just removing the skin file. This would require more extensive refactoring of your css files, however (something like splitting more information into separate skin files which could then be left out of the build process), so for the first version of this pull request I have started just by removing the main skin file.