Handle when types can't be loaded from assembly

As per #22 - This looks like a slightly better fix: http://haacked.com/archive/2012/07/23/get-all-types-in-an-assembly.aspx

``` csharp
    try
    {
        return assembly.GetTypes();
    }
    catch (ReflectionTypeLoadException e)
    {
        return e.Types.Where(t => t != null);
    }
```
