summary: updated build.xml and tests/build.xml.

build.xml and tests/build.xml:
  updated general format to match what is generated in a new project.
  moved targets common to both to build_common.xml.
  replaced some exec calls (perl, mv) and inline javascript with ant tasks (including extensions from Android and ant-contrib).

build.xml:
  fixed indentation.
  privatized targets that don't need to be called directly from command-line.
  renamed some properties.

tests/build.xml:
  renamed test project from K9 to K9Tests.

tools/ant-contrib.jar:
  new!

.gitignore:
  added note about local per-repo rules.