Fix links from the VOTable validator

As @pllim pointed out, the links in the VOTable validator output that point back to the `io.votable` docs needed to be updated to reflect the reorganization of those docs.

@eteq: You have want to have a look since this changes `online_docs_root`.  I think this is fine, but I'm not sure what, if anything else, uses it.  Note that while `docs.astropy.org` redirects to `http://docs.astropy.org/en/v0.2/`, subdirectories do not redirect (i.e. `http://docs.astropy.org/io/votable/` is simply a 404).  So to be a real document root that could have further subpaths appended to it, we need to include the language and version.  I also had to move `online_docs_root` from `astropy.utils` to `astropy` to avoid a cyclical impot problem.  This could also be resolved by turning `online_docs_root` into a function if preferred.