Authentication fix + download-all

Hi chipx86, the following changes roll up what @tgross has done, make the requests a little more resilient to errors, and add a "download-all" command to pull an entire library. Four interesting changes:

* `download-all` is a new command. I think that's fairly easy to understand.
* `--dry-run` keeps `download-all` from actually running, which can take a long time.
* `--start-at-artist` uses the `GREATER_THAN_OR_EQUAL` search operator to start midway into a list of artists.
* `download-all` skips existing files.

Here's an example from inside the cloudplaya directory:

    python main.py download-all --dry-run --out-directory=/tmp/ --start-at-artist "Fartbarf"