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 not defined, use
"Mozilla/5.0(compatible; CCTools 4.2.0 Parrot; http://www.nd.edu/~ccl/)"
as the User-Agent header of the HTTP request. If "HTTP_USER_AGENT" is
defined as http_user_agent_value, the User-Agent header of the HTTP request will be
"Mozilla/5.0(compatible; CCTools 4.2.0 Parrot; http://www.nd.edu/~ccl/
http_user_agent_value)".

Fixes #314.