Use debugfunction as a callback for setting raw request headers (fixes #63).

More details here:
http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTDEBUGFUNCTION
Verbose must be set to true to use custom debugfunction.
Get specs passing to account for verbose always being true.
Add request_headers to Easy#to_hash.
Add specs for request_headers.