Move docker version introspection to a sub-package.

This facilitates the refactoring of commands.go.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Move even more stuff into dockerversion

Also, use it in all the places. :)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #74 from tianon/more-separate

Move even more stuff into dockerversion
Fix the one spot I missed dockerversion

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #75 from tianon/fix-my-broken-eagle

Fix the one spot I missed dockerversion
Fix the tests, too

Seriously.  There's not much codebase left we haven't touched.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #76 from tianon/fix-broken-eagle-tests

Fix the tests, too
Merge pull request #4074 from shykes/separate-version

Move docker version introspection to a sub-package.