b:message rendering can be improved

[NicolaIsotta]

• a null may appear in the generated class attribute
• if there is a col- class, wrap the alert in an outer div (see Some components render invalid BS markup with col-XX attributes #675 )

I've already prepared a PR

[stephanrauh]

@NicolaIsotta The change looks OK to me. Thanks!

I've got two questions:

• Do we have to add something to the documentation? (http://www3.bootsfaces.net/Showcase/forms/messages.jsf)
• What about b:messages? Should we improve it in the same way as b:message?

[stephanrauh]

Done and documented.