lib: integrating Mail module inside Curl module

- Adding new request Class named CurlMailRequest.
- Adding new response Class named CurlMailResponse.
- Updating sample.
- Removing CurlRequest Url, Headers, Datas attributes : Too specific for
  this class. Now defined directly in children if needed.
- Setting Curl instance attribute directly to null.
- Adding new intern abstract class to instance registering named
  CurlCallbacksRegisterIntern.
- Removing CurlCallbacks inheritance from CurlRequest, now directly in
  children.

Signed-off-by: Matthieu Lucas <lucasmatthieu@gmail.com>
lib: fixing visibility of intern use only Curl Response class

Signed-off-by: Matthieu Lucas <lucasmatthieu@gmail.com>
tests: updates sav for examples/curl_mail.nit

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
examples: clean up empty lines in curl_mail.nit

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>