use of nullptr causes compilation error with gcc 4.6.3

```
Similar to issue 14, the use of nullptr causes compilation error with gcc 4.6.3 
:
./libs/libember/glow/GlowInvocation.hpp:117
and 
./libs/libember/glow/impl/GlowInvocationResult.hpp:129

```

Original issue reported on code.google.com by `benoit.q...@lawo.de` on 22 Jul 2013 at 9:12
