Matlab toolbox cleanup

This PR is the first step in the cleanup/reorganization of the Matlab toolbox.

- remove `demo/*` and `functions/*`
- clean all references to the deprecated gateway
- move `helper/*` functions under `src/`. Removed a couple of deprecated/useless helper functions
- fix `release-zip` target to account for the new folder structure. Copy all jars in `libs/` directory.
- review `toJavaList()` and `toMatlabList()` function. Allow casting in toJavaList, e.g.
```
ids = toJavaList(1:10, 'java.lang.Long')
```.
 Wrote corresponding tests under `test/unit`
- extend `toMatrix()` helper function to be applicable to stacks loaded with `getStack()`. Wrote unit test under `test/unit`
- fix failing `TestLoadOmero` test
- add `test-unit` target to run all unit tests. Can be tested with
```
MATLABPATH=path/to/xunit ./build.py -f components/tools/OmeroM/build.xml test-unit
```

/cc @jburel, @imunro