Fixing ReadTheDocs reading docstrings

Hey,

At the moment ReadTheDocs still isn't reading the docstrings correctly. The reason is due to the strange behaviour in Sphinx in that it will run any code it scans that isn't embedded in functions or classes (or in the if __name__='__main__' loop, due to the way import works in Python). The code causing the trouble was in datasets.py, where it automatically tries to read the JSON data, on the local machine this runs fine, but ReadTheDocs doesn't actually copy the data, and so this then threw errors breaking all the docstring parsing.

This little fix seems to fix it on my test ReadTheDocs repository. This would have to be merged in to master to have any effect on the ReadTheDocs repository.

You can see what I mean here: 
The fixed repo: http://gpytest.readthedocs.org/en/latest/
The broken repo: http://gpy.readthedocs.org/en/latest/

The module pages are blank in the latter.

Cheers,
James