Add custom message to `withCallback()`

[adamkelso]

Currently, the API for withCallback() accepts one argument, the closure to filter the history items. A second one should be added so that the user can pass a more helpful message for what their closure is actually checking.

// Before
$expectation->withCallback(function ($history) { 
   /* something */ 
   return bool;
});

// After
$expectation->withCallback(function ($history) {
    return bool;
}, "Failed asserting that the Rhinos are just overweight unicorns.");

[adamkelso]

Completed in release 1.5.1.