0.8.0

* fix build on Windows/msvc (Nicolas Ojeda Bar)
* fix debugfunction invocation
* require libcurl >= 7.17.0
* less mallocs per handle
+ CURLOPT_USERNAME CURLOPT_PASSWORD CURLOPT_LOGIN_OPTIONS CURLOPT_CONNECT_TO
+ build cmxs
* fix build tests with OCaml 4.06.0