Add small improvements to `legend` normalization.

Remove the default padding. In theory, the correct normalization
would be to add the padding to IE6/7 rather than remove it from
all browsers. However, the most common use case is going to be
for legends within unstyled fieldsets, and the presence of 2px
of horizontal padding is likely to be unexpected.

Allow text wrapping in Firefox 3. Modify the default white-space
value to allow long legends to wrap. No simple fix to allow this
in IE6/7/8.