NoneType error on bbox
Hi,

Trying to workaround FontAwesome awkwardness, i hit a bug with bounding box management. Here is the output:

``` console
$ icon-font-to-png --css my-font-awesome.css --ttf my-fontawesome-webfont.ttf ALL
Exporting icon '500px' as '500px.png'(16x16 pixels)
Traceback (most recent call last):
  File "/home/bersace/.local/share/virtualenvs/pandocker/bin/icon-font-to-png", line 16, in <module>
    command_line.run(sys.argv[1:])
  File "/home/bersace/.local/share/virtualenvs/pandocker/lib/python3.5/site-packages/icon_font_to_png/command_line.py", line 161, in run
    color=args.color, scale=args.scale)
  File "/home/bersace/.local/share/virtualenvs/pandocker/lib/python3.5/site-packages/icon_font_to_png/icon_font.py", line 155, in export_icon
    border_w = int((size - (bbox[2] - bbox[0])) / 2)
TypeError: 'NoneType' object is not subscriptable
```

``` python
52             if bbox:                                                         
153                 icon_image = icon_image.crop(bbox)
154                                                                        
155  ->         border_w = int((size - (bbox[2] - bbox[0])) / 2)
156             border_h = int((size - (bbox[3] - bbox[1])) / 2)
157                                
158             # Create output image
159             out_image = Image.new("RGBA", (size, size), (0, 0, 0, 0))
160             out_image.paste(icon_image, (border_w, border_h))
```

I don't understand why there is an `if bbox` as if bbox can be None, but `border_*` lines requires `bbox` to be defined. Can you give me a clue ? I can send you a patch.