Allow keeping the result of an artifact build without cleaning it up

[becw]

Addresses #109.

I had to fix two bugs while working on this:

• The composer location stuff from Allow configuring what command to use for running composer in the artifact #110 was failing because it needed the full path to composer (even when I didn't configure the composer.composer property)
• The artifact-resetState target was failing on some of the cleanup <exec> statements

Other than that, this changes the artifact build process so that instead of prompting the user with Push artifact changes to the '${artifact.git.remote_branch}' branch? y,n, it offers this prompt:

Push artifact changes to the 'artifact-test' branch?
  0: push
  1: keep
  2: discard
 [0]

This defaults to pushing if you hit enter.

• push pushes the artifact to the remote
• keep doesn't push, but leaves the generated artifact in place so that you can examine it
• discard reverts to the previous version of the artifact

[becw]

Merging this in order to get these bugfixes, and cut another alpha tag.