Functions in the HTTP API:
http.checkURL( url )
http.request( url, [postData], [headers] )
http.get( url, [headers] )
http.post( url, postData, [headers] )

The HTTP API may be disabled in ComputerCraft.cfg
A period of time after a http.request() call is made, a "http_success" or "http_failure" event will be raised. Arguments are the url and a file handle if successful. Arguments are nil, an error message, and (optionally) a file handle if the request failed. http.get() and http.post() block until this event fires instead.
