Just a tidbit of refactoring.

Add --use-binary option which allows user to choose which binary
should be placed in the image if go get builds multiple binaries.

This change also introduces some changes in naming - when project
example.com/foo/bar has binaries bar and quux, then for quux the
created image will have a filename bar-quux.aci, name in manifest will
be example.com/foo/bar-quux.

Add --asset option which takes two paths separated with OS specific
path separator (colon on Unices, semicolon on Windows). The first path
specifies a path inside produced image, second part - the path from
local filesystem which will be copied into image.

We handle only regular files, directories and symlinks that does not
point outside the asset.

This is strictly developer option to be able to view the contents of
/tmp/goaciXXX directory, when something goes wrong.

It supports getting information from git, hg, svn and bzr.

The goaci.go already looks like a kitchen sink with two year old
leftovers. Let's clean it up a bit.

If a symlink target is an absolute path still pointing somewhere
inside the asset it sits in then preserve the absoluteness even in
rootfs. That requires a bit of path munging.

Add two placeholders - GOPATH and PROJPATH. GOPATH placeholder is
obvious. PROJPATH placeholder will be replaced with absolute path to
project go get is building. Usage is simple:

--asset /assets:<PROJPATH>/assets

Split out some code to separate functions with hopefully
self-explanatory names. The additional aim is to have function fit
into single screen.

Remove some more clutter from goaci.go.

We are using flag package for option parsing, so stop using os.Args.
We can take the project from flag.Args after ensuring that it has one
and only one string.

This basically rewrites goaci.go into many smaller and clearer
functions. It also removes all uses of die function, so the deferred
removal of /tmp/goaciXXX is actually executed (die used os.Exit()
which prevented that).

That simplifies the code a lot for a short while. Later some of it
will probably come back when we implement checking for dangling
symlinks after assets are copied.

These usually are filenames, so it's good to delimit them with quotes
in case there are spaces in them.