Patch 6 -- Replace invalid use of legend tag

Problem:
In the_bootstrap_comment_form_default a legend element styles the comment form title. W3C says the legend element should only be used as the first child of a fieldset element. http://www.w3.org/TR/html-markup/legend.html

Changes:
Replace the legend element with span.legend.
Add .legend to style.css (copied from Bootstrap legend element style). 

Warnings:
My git-foo is weak, so I may have packaged this wrong. Make sure the only changes coming with this pull are to functions.php (the_bootstrap_comment_form_defaults) and style.css (add .legend in classes section).

People who customize Bootstrap.css with non-standard font-size, line-height, color, etc. will need to override the default .legend styles. 

This could create some challenges with the included Bootswatchr themes.

It is possible that the full .legend styling is not needed. For example, testing suggests the width, padding, and color attributes don't affect appearance. Removing these may prevent some of the potential Bootswatchr problems.