devserver: general cleanup

* Unified logging API via module/class-local _Log() calls.

* Renamed modules: devserver_util --> common_util (this is the common
  util module, and it saves a lot of characters in many places);
  downloadable_artifact --> build_artifact (we don't have any
  non-downloadable artifacts); buildutil.py --> build_util.py (for
  uniformity)

* Reorganized Artifact (now BuildArtifact) class hierarchy to properly
  reflect the inheritance chains.

BUG=None
TEST=Unit tests run successfully

Change-Id: I91f3bd7afe78cc1e2be10b64640515402fb6184b
Reviewed-on: https://gerrit.chromium.org/gerrit/33755
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Ready: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/autoupdate_unittest.py b/autoupdate_unittest.py
index 2302cf4..2010bbf 100755
--- a/autoupdate_unittest.py
+++ b/autoupdate_unittest.py
@@ -265,5 +265,5 @@
     self.assertFalse(au._CanUpdate('0.16.892.0', '0.16.892.0'))
 
 
-suite = unittest.TestLoader().loadTestsFromTestCase(AutoupdateTest)
-unittest.TextTestRunner(verbosity=3).run(suite)
+if __name__ == '__main__':
+  unittest.main()