`pb_download` tries browser download URLs first to reduce ratelimiting

[tanho63]

Resolves #109

Changes

Adds download_url to gh_download_asset and then tries to download from there first, falling back to the previous API download and download redirection after that if for some reason it doesn't work.

Other changes

• Documentation for pb_download and pb_upload copyedited
• Adds comments and some light refactoring to pb_download()
  • assertion to ensure either dest = length 1 or length of dest = length of files to download
  • minor readability/simplification improvements
• Updates to test infrastructure
  • Now checks test token (TAN_GH_TOKEN) to see if it has push permissions to the test repositories (tanho63/piggyback, tanho63/piggyback-tests, tanho63/piggyback-private) which should reduce erroring on this step :crossed_fingers:

[cboettig]

p.s. @tanho63 just wanted to say it's also such great fun working with you!

[tanho63]

Thanks, pleasure is mine! I think this is one of the first non-football-specific packages I've helped out with and I'm glad to keep pushing it forward where helpful