0.11 devel docfix http response status

Minor documentation with regard to this google groups post: https://groups.google.com/forum/#!topic/cpp-netlib/UtrdJmnGO-w. 

Fixes HTTP response status wrapper return value documentation.

Also fixes some formatting issues along the way: trailing white spaces, tabs instead of spaces ("Use spaces instead of tab characters." http://cpp-netlib.org/style-guide.html#formatting).

However, this pull request does not include the built resulting documentation with sphinx as I've seen others do.