Use relative paths for exclusion matching in weaver.

Hi, I've run into a problem while building kilim from source. The weaver would not process any of the classes after they have been compiled, specifically this line in `build.sh` didn't work:

```
java -ea kilim.tools.Weaver -d ./classes -x "ExInvalid|test" ./classes
```

It turns out that the problem was that the regular expression of the exclusion filter was matched agains the absolute path of each `.class` file. And because I checked out the repository into a folder called `kilim-test`, all files would be excluded by the filter.

I've fixed it by modifying  `DirIterator.getFileName()` so that the paths it returns are no longer absolute but relative to the directory specified on the command line.