Fixes build issue for Boxed_Value

This ixes the problem, that one has to include chaiscript.hpp to build
compilation units, which only use Boxed_Value and boxed_cast.
You can now just include boxed_cast.hpp. This decreases build time
significant.
Merge pull request #87 from bmario/ChaiScript_5_0_CPP_11

Fixes build issue for Boxed_Value