Retry package based on Sean's composition design with essential logical refinements / fixes

Added more tests, custom error structs and fixed logic in essential parts: For this introduced a tristate as decision output of retrier aspecifiedd evaluator for composed strategies.

We use three states, Yes, Maybe and No. To evaluate composed strategies:

```
// We retry (output Yes) if one or more retriers evaluate to Yes and none says No 
// To output Yes some can also evaluate to Maybe as long as one evaluates to Yes. 
// If all evaluate to Maybe we will return Maybe.
// If one evaluates to No we'll return No regardless what the others returned
```

To optimize the evaluation by the strategies.Retry function we make use of arithmetic properties of the values representing the states.


For the use-case present it may not be as important, just to identify the error as what it is. So thats why we use custom error structs. So if we make custom error structs, why not pass all information possible?