add fern icon

[mfrasca]

case 'Thelypteridaceae':
case 'Thyrsopteridaceae':
case 'Woodsiaceae':
    // doc.icon = 'fern'; // there is no such icon yet
    break;

[mfrasca]

I grabbed a couple of fern icons from open clip art, marked as public domain, and I'm working on them. but in the end we need to reduce them to the tiny size in glyphicons-halflings.png.