Adding header not working #373

hi,

I was looking at this pull request:
#291

I went on to test it and did the following:
header = section.header
header.text = 'foobar'
Reference for code:
http://python-docx.readthedocs.io/en/latest/dev/analysis/features/header.html?highlight=header

Section object has no object [header]

I think this feature is implemented but not included. Please correct if I am wrong.

http://python-docx.readthedocs.io/en/latest/api/section.html#docx.section.Section

I am not sure as it says it is active. I think it means work is been done.