Implement proxy variables for curl_setopt

Updated File.php since discussion on issue https://github.com/simplepie/simplepie/pull/357#issuecomment-39635250
Implement proxy variables for curl_setopt (part 2)

Linked with https://github.com/simplepie/simplepie/pull/359
Update SimplePie.php
Merge remote-tracking branch 'proxy/patch-1'
Merge remote-tracking branch 'proxy/patch-4'