Update build.xml

* Added global properties;
* Added missing classpath IDs to "compile" target;
* Added new "init" target;
* Added new "clean" target;
* Added new "doc" target;
* Set "includeantruntime" to false in javac
That's caused by a misfeature introduced in Ant 1.8. It defines whether to include the Ant run-time libraries in the classpath; defaults to yes, unless build.sysclasspath is set. It is usually best to set this to false so the script's behavior is not sensitive to the environment in which it is run.

It compiles fine on MacOS X 10.8.2. Ant 1.8.2.
Merge pull request #6 from gvarisco/patch-2

Update build.xml