Refactor Ant buildfile #3

I ended up refactoring the file quite heavily so the diff is a bit messy, sorry about that.

I think the easiest way to build the docs on the command line after this commit is to do something like:

```bash
$ export DITA_HOME=/path/to/dita-ot
$ ant -lib $DITA_HOME -lib $DITA_HOME/lib all
```
It would be possible to avoid having to set the CLASSPATH or using the `-lib` switches at all, but it would probably make the buildfile a bit too complicated.