Dataset object download_info and readme

[magdalenafuentes]

Looking at #300 I realized that many dataset loaders don't have the readme implemented (I checked a few), but it appears in the docs of the Dataset object. Should we implement it or update the docs?

Also, in the docs it appears that download_info is visible, but it currently isn't _download_info. We should make it visible right @rabitt ?

[rabitt]

On the readme - this is the one thing I removed when updated to the new new API. I didn't find an easy way to load the docstring from the module itself from within the module. I wasn't too upset to remove it though - for me, its why the documentaiton exists! But if you want to still have something in the code, maybe we can have a Dataset object defined .readme for all Datasets which prints a link to the readthedocs page.

On download_info - good catch! It's in the documentation for the Dataset object (by mistake). I'd removed it from being since it's meant to be part of the download function. I was afraid of confusing people when download_info is None, making them thinking there's nothing to download - but in fact those are the most perfectly downloadable datasets! Either way we need to fix the inconsistency!

[magdalenafuentes]

I like the dataset.readme printing a link to the docs or something like that, but I'm ok either way! @nkundiushuti thoughts?

Also ok either way with the download_info. It might be slightly better to be hidden.

[rabitt]

This is fixed in #410