Message editor API

[kurotych]

• push_top/push_bottom(header_name)
• pop_top/pop_bottom(header_name)
• add_header(header_object) or add(header_name, header_value)
• get_header_value(s)?
• add_parameter?

[kurotych]

It seems I'll need to allocate new memory in the serialize() function.
But I should have possible to change the message (add/remove header/parameters) without additional copying.
Am I need a new object for this stuff or implement this functionality in existing structures?