changing visibility of the Curl cleanup method
Adding mail feature, and updates to CURLSList and Callbacks methods
- Adding new way of callback to read datas.
- Adding/Updating CURLSList datastructure, now shipped with to_a method,
  and collection has been filled by a to_curlslist method.
- Adding mail feature, multiple writable attributes (headers,
  recipients, senders, subject, body etc) and useful methods.
- Updating Makefile to insert mail executions / tests.
- Updating Curlunit to add some CURLSList related tests.
- Adding a mail sample.
Introducing calls to copy_from_native to prevent for memory releasing
Changing calls to String::+ by superstring {}, because no more exist
removing useless cast
Updating Makefile