Fix #514: large number words in English

[Quuxplusone]

This patch addresses #514 in the most straightforward way. I don't have any way to actually verify this against Mathematica's names, though; I think somebody should do that before this is merged.

EDIT: Okay, Hans Havermann has verified it:

The 101 names at the bottom of /pull/517/files ('ten thousand', 'ten million', 'ten billion', ... 'ten centillion') do indeed match their Mathematica counterparts.

I'm not sure what effect this has on number names in non-English languages. These changes don't break any existing tests, but I believe all that means is that there's no existing test coverage for any of these names in non-English languages. (There was no coverage for English itself, before this patch.)

[Quuxplusone]

flake8 complains about the line lengths on those new comments. I'll wait for an opinion from the maintainers on how to go about fixing/quieting that. (E.g. two options are just to remove those comments, or to replace "-gintillion" with "-gx" throughout the comments; but neither of those strikes me as an obviously beneficial change.)