add ability to retry on custom conditions

[elennick]

Currently retry4j only supports retrying on a set of exception types or values being returned from the callable logic. There also needs to be a way to retry on other conditions such as if a exception is returned with a certain value inside of it... for example:

HttpClientErrorException is thrown by apache HttpClient, you may want to retry only when ex.getRawStatusCode returned a 5XX error code and not a 4XX error code from