A bit of cleanup / infrastructure

@lpaioro Here's some cleanup and I've added a first unit test. You can comment on things you don't like on github and tell me to change them. Or if you like the changes you can pull them into your branch by hitting the merge button on github.

The unit test fails. You can run it via
```
python setup.py test --package vo
```
from the top-level astropy folder to see what the problem is.

@astrofrog If you have time to have a look that would be good.

I guess the first goal should be to get this unit test and the `sampy` command line tool working. Then we can do further re-structuring, e.g. I probably `core.py` should only contain the classes and the script should be in a separate file.