Add a User-Agent header to prevent throttling.

37signals appears to throttle requests that do not contain a 
User-Agent header.

This error shows up in the last file downloaded when Campfire Archiver crashes:

    You have exceeded 50 missing_useragent_header requests in a 
    10-second period. Please wait 10 seconds before retrying.

Setting a User-Agent header allows for full download.

This fixes jbt/campfire-archive#1