add more complete Less support

This patch adds Less support; it is based on the patch from @lexaficus, with some bug fixes added. The other pull request is currently on hold due to a merge issue, so I decided to create an new one for these changes.
This patch adds the following features/fixes that were not in the original patch:
- When using Less, there is a prompt to let users decide whether to use the Less  version of Bootstrap, or the regular CSS version
- Bootstrap stylesheets were included twice in the original patch. In the Less stylesheet via (@import .../bootstrap.less), and also in index.html via the wiredep plugin (bower:css). This is fixed with this patch.
- Wiredep is also better integrated with Less in this patch. I added a bower:less in the generated Less stylesheet.
- Other cleanup in _Gruntfile.js (e.g. The copy:styles task is not generated when using Sass or Less, fonts files are assembled in the default bootstrap folders: /fonts/ when using the Less version of Bootstrap, /fonts/bootstrap/ when using the Sass version of bootstrap; Having font files in /dist/bower_components/... is very out of place in a distribution folder.)