Normalize fieldset border. Fix #34

IE renders rounded corners on fieldsets by default.

All browsers - even those that use the same border value of
2px groove threedface in their UA stylesheet - have different
final rendering colors, widths, and joining of the border.

The chosen value of 1px solid #c0c0c0 takes the most common
visual border width (IE, Firefox, Opera), removes the problematic
groove width value, and takes the computed color value from Chrome.