Have Parrot with HTTP send User-Agent.

Enable Parrot to send a User-Agent string when making an HTTP request.
If environment variable "HTTP_USER_AGENT" is defined, use its value as
the User-Agent header of the HTTP request; otherwise, use "Mozilla/5.0
(compatible; CCTools 4.2.0 Parrot; http://www.nd.edu/~ccl/)" as the
User-Agent header of the HTTP request.

Fixes #314.