Add license line to each file

All code in this package is BSD licensed?

If so, could you add this line to the top of every file (see [astropy guidelines](http://docs.astropy.org/en/latest/development/codeguide.html?highlight=license#coding-style-conventions))?
```
# Licensed under a 3-clause BSD style license - see LICENSE.rst
```

Then it's clear that `pyvo` is license-compatible with `astropy.vo` and `astroquery`.