Resume downloads

[bcipolli]

During model download, my Internet connection was cut. On re-running models/download_models.sh, I noticed that:

• Files that were downloaded fully got downloaded again.
• Files that were incomplete got downloaded from scratch.

Adding the -c flag to wget works around each of these issues:

• If a file has been downloaded fully, wget figures that out.
• If a file needs to be resumed, wget resumes at the proper location.

I also noticed that downloaded files weren't ignored by git, so I added the models directory (excepting the download script) to .gitignore

[jcjohnson]

Thanks!