Modify trace api

@mortenvp I finished the unit tests and fixed the todos as good as i could. Please let me know what you think? :)
You don't have to store.. you can just cast it to a bool.

Ok, I see that point, but I think the other version is easier to read.. and as long as the test passes there is no problem with just the bool. When it starts failing you can add the print.

Actually we could add to the ``stub::call`` class an ``operator<<`` then you could do something like:

```
SCOPED_TRACE(testing::Message() << stack.write_trace);
EXPECT_TRUE((bool) stack.write_trace.expect_calls().with("input_symbol_data", expectd_hex.str());
```
And it would only print the write_trace calls if the test fails.