Make jar runnable and improve parsing error reporting

Hi,

First thank you for creating this small project, this validator works better than other GUI, especially when working with several files.

In my changes: 
- [x] I added build stuff to make the jar file runnable by itself without having to change the classpath.
- [x] I fixed the following build error that would appear when building twice without explicitely calling `ant clean`:

```
jar:

BUILD FAILED
D:\work\xsd-validator\build.xml:76: A zip file cannot include itself
```
- [x] I added a more precise exception handling that also reports the line and character number to make error finding easier
- [x] I updated the jar in lib/jar, although I think it's not a good practice to ship the binary in the version control system, because you have to re add it every time it's changed ;)

For future improvements, I will try to see if there is a way to validate the whole document and report all the errors in one log instead of failing at the first error, but I'm not sure if it is possible.