Fix fieldset width in Chrome and Firefox

Fixes an issue in Firefox [1] and Chrome [2]. The implicit width of
`fieldset` can be determined by the width of descendants rather than the
parent container. This change (adding `min-width:0`) makes the
`fieldset` behave like a `div` in this regard.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=504622
[2] http://stackoverflow.com/questions/8084343/google-chrome-fieldset-overflow-bug

Close #272