Added Zlib missing metthods.

- Added 'rubinius/ffi' require.
- Removed all references to Rubinius::ZLIB_PATH.
- Started to use z_* conventions for FFI attached methods from Zlib.
- Added Zlib.crc32_combine.
- Added Zlib.adler32_combine.
- Added Zlib.zlib_version.
- Removed Zlib.do_checksum.
Added methods to GzipWriter

- Added print, printf and puts and delegated to IO object on GzipWriter.
- Zlib.crc32 and Zlib.adler32 now raise RangeError.