DRYing up creation of X-Registry-Auth header

Conflicts:
	image.go
Create tar stream for user with specified context dir
Removing support for go1.1
[TRIVIAL] removing extra comment line [ci skip]
Removing comment per feedback
Oops, adding to headers instead of reassigning
Allowing X-Registry-Config header to passed in via the environment
Merge remote-tracking branch 'origin/master' into provide-build-tar-stream-for-user

Conflicts:
	image.go
Adding tests related to building from a contextdir
Adding support for X-Registry-Config header (and corresponding test)
Merge remote-tracking branch 'origin/master' into provide-build-tar-stream-for-user
Updating test after merging master