Improve init script

[rjust]

The init script should be improved in three ways:

1. When re-executed, the init script skips downloading dependencies (unless there is a newer version) but unzips archives and sets symlinks. Some calls to unzip force replacing existing files, whereas others query the user -- the latter is undesirable.
2. The init script should fail fast. Specifically, if unzip or ln fails, the script should stop immediately.
3. The init script should provide a "force flag", which deletes all downloaded/initialized dependencies and performs a non-incremental initialization.
4. The init script should download ant and properly configure it. All calls to ant in D4J should use the configured version of ant. (Currently, D4J uses Major's ant version to compile all bugs, but possibly a different version (system installation) when running external tools. This is confusing and makes debugging ant configuration issues really hard.