Declspecs for stl member variables

These have me in a spin:

```
 warning C4251: 'ecl::StandardException::message' : class 'std::basic_string<_Elem,_Traits,_Ax>' needs to have dll-interface to be used by clients of class 'ecl::StandardException'
```

This is a common warning when you have something as simple as a `std::string` inside an exported class. Not sure of the right way of dealing with these.
