Removed useless file: curl_handle. The definition of curl_handle was, fundamentally, useless.
Removed useless file: curl_message. Its definition and implementation has been moved into curl_multi as nested class with inline methods.
Added classes and methods documentation.
Use of unique_ptr to avoid explicit memory deallocation.
Fixed all the copy constructor and assignment operators.
Some small methods has been implemented as inline, to improve speed.
Other small bug fixes.