Test for CURLSSH_AUTH_AGENT

CURLSSH_AUTH_AGENT is introduced in curl 7.28. Ubuntu 12.10 comes with
curl 7.27. If not defined we use CURLSSH_AUTH_PUBLICKEY instead.
Add curl debug function

Record curl debug information. The information is printed in case a download
results in a non HTTP related error. Usefull to debug protocols with authentication.
Merge branch 'download-debug'