Add add_header and unset_header + docs

This commit squashes commits 53c2e28d...3588a9a2

Credits go to Olivier Favre (@ofavre) and Hauke Lampe (@lampeh)
who did all the hard work
s/add_header/header_add/ & s/unset_header/header_unset/

As per PR Feedback
Merge branch 'customHeaders'