Fix WT2, restructure splits, and add WMT14

[jekbradbury]

Also adds nonoptional dependencies to setup.py so pip install torchtext will fetch them.

The only known possibly-breaking change (I'd call it a bugfix) is that Dataset.splits will now use os.path.join rather than concatenation to join provided paths.

[bmccann]

Nice; the added download functionality is great, especially for Google Drive. Is there any way to unify the splits for the other datasets even more?

[bmccann]

It is a little odd to have the WMT14 be the preprocessed version from Google, but I suppose we can worry about that later if we want to add other versions of WMT.

[jekbradbury]

Agreed, though the others are a lot more complicated and this one allows direct comparison with Google's GNMT/Transformer results

[jekbradbury]

Also the confirm_token thing sits there for easily five minutes before actually downloading?? I don't know what's up with that; the logic is exactly the same as what Google uses (and it looks like they got theirs from StackOverflow anyway)

[jekbradbury]

Anyone know what to do about the bogus flake8 error? I think I've seen it before but I don't remember what if anything makes it go away (other than #NOQA)