Test-matlab fixes

This PR fixes a couple of bugs in the `test-matlab` ant target, some of them raised by @manics in #312:

- add a description to `test-matlab`. The target sould be listed by `ant -p` now
- fully remove the reader objects after closing them to get rid of java warnings raised when TestCheckJavaPath() was run after TestBfsave() or TestBfGetPlane()
- fix `-nodesktop` and `-nosplash` options spelling
- add the Matlab functions and artifacts to the MATLABPATH environment variable when running test-matlab. The only path that needs to be specified is the one to `xunit` module since the latter is not included in bioformats.git. Tested locally with the following command:
```
MATLABPATH=~/code/lccb/extern/xunit/ ant test-matlab
```