RadioField renders render_kw

[azmeuk]

This pull request comes after the accidental closing of #628 and fixes #490. I cherry-picked the commits of #628 so the author is remembered.

Basically this patch renders the RadioField render_kw parameter in both the generated <ul> tag, and the <input type="radio"> tags inside. Before it was rendered only in <ul> and not in the <input>.

Passing disabled=True in the render_kw will keep adding a disabled parameter in <ul>. This may look wrong but this is harmless and not worse than before.

I am not sure how to improve this. For instance, how to ventilate render_kw items so some are displayed in <ul> and other in <input>? What about class, should this be rendered in both?

I think the situation is still unsatisfying with this patch, but at least it goes in the right direction. It is a good-enough PR in my opinion.

What do you think?

[azmeuk]

Maybe we could only display in <ul> the attributes that are part of the HTML Global Attributes.