Implement SDL_RWops in Rust as sdl2::rwops::RWops

* fix compatiblity with newer rust (std::libc => libc)
* add RWops struct (implements ``Drop``, ``Reader``, ``Writer``, ``Seek``) 
* fix #38 , now calls close() in drop()
* use ``close_on_drop`` mentioned in #38

TODO:
* add ``RWops::from_mut_bytes`` (``SDL_RWFromMem``)
* a close method?