include <algorithm> for std::swap

This avoids compile error on OS X 10.8 with libstdc++:

```
In file included from /Users/dane/projects/mapnik-packaging/osx/out/packages/luabind/build/test/test_headercompile_handle.cpp:2:
/Users/dane/projects/mapnik-packaging/osx/out/packages/luabind/luabind/handle.hpp:100:10: error: no member named
      'swap' in namespace 'std'
    std::swap(m_interpreter, other.m_interpreter);
    ~~~~~^
/Users/dane/projects/mapnik-packaging/osx/out/packages/luabind/luabind/handle.hpp:101:10: error: no member named
      'swap' in namespace 'std'
    std::swap(m_index, other.m_index);
    ~~~~~^
2 errors generated.
```