Don't issue HTTP requests without a 'user agent' header

[jetvillegas]

WP Engine (and perhaps other blog hosts) reject HTTP requests that don't have a user-agent field in the header. Planet Venus issues such requests for feeds when spider_threads > 0.

[jetvillegas]

Actually, what the server is rejecting is the default user-agent sent by the outdated httplib2 in the venus tree:
"user-agent: Python-httplib2/$Rev$"

The latest httplib2 seems to have fixed this issue.