Replace auto_ptr with unique_ptr.

This fixes deprecation warnings from Clang.

Adapted from the patch to fhoefling/luaponte:
https://github.com/fhoefling/luaponte/commit/19d9a14fd92127dd00399d9da88e54f5b92decd4
Remove luabind_cxx11.h.

This compatibility header isn't needed on MSVC either, so remove it
entirely.