Fix bootstrap3 icon files

The initial impeteus for the commit is that the index.liquid file of the bootstrap3 generated previously referred to 4 non-existent files. I don't really think the placeholder bootstrap icon is that important, but assuming it's included in the generator, it might as well be included correctly.

Changes:
- Changed reference to _favicon.ico_ to _favicon.png_
- Renamed the touch icons to meet the standard naming convention (http://mathiasbynens.be/notes/touch-icons)
- Changed the touch icon references to reference the new, existing files.
- Changed the order of the touch icons for compatibility with  legacy devices (http://mathiasbynens.be/notes/touch-icons)