Implement way to ignore if already downloaded or is completed

[edelciomolina]

Create a way that we can decide if the files already downloads (since really fully completed) have to be downloaded again or not. I noted on some tests that the download always starts again.

Ideas:

• Check local file header "last-modified" and compare with the remote file is newer.
• Check local file header "content-length" and compare with the remote file to know it was download correctly.