* Add ability to set authenticated user in request header
Some web applications provide the ability to authorize users based on
the authenticated from Basic Auth. This patch provides a way to set a
key to which the authenticated user can be set in the Header.
For example, if I set `HeaderValue = "X-WebAuth-User"` and authenticate,
my application will be able to read my user name from that header and
provide me with the proper access.
This fixes#802
Prior to this fix the documentation for the swarm example included
syntax that would fail with the following error.
`Error : flag needs an argument: --docker.domain`
This fix specifies flags using the `=` between the flag name and value.
Tested on: Docker version 1.12.2-rc1, build 45bed2c, experimental
