Commit logspout binary to repository; update Dockerfile reference and Ma...

...kefile

Updated Makefile:
 - to have a stage/logspout target which copies the build output to the stage directory
 - to depend upon the stage/logspout when building the container
 - to force the OS/ARCH to be linux/amd64 (prevent Mac users from destroying the world)

Updated Dockerfile:
 - ADDs from the stage location

I spaced on putting this into a branch, hopefully, it's useful on master.

I took a stab at updating the Makefile and Dockerfile; feel free to spindle, fold, and mutilate as you like.

I believe this addresses the request you made in https://github.com/progrium/logspout/issues/5