Fix form object memory leak

Make sure to free the first form entry with curl_formfree
http://curl.haxx.se/libcurl/c/curl_formadd.html

Also, switching the form pointers to normal ones, since we're manually freeing
the memory associated with them.

fixes typhoeus/ethon#43
After calling curl_easy_escape call curl_free

I think this all could also be simply replaced with CGI.escape, but we can
change that later.

fixes typhoeus/ethon#44