Added hspec tests

Hi, 

As we discussed in [here](https://github.com/Soostone/retry/issues/4), I factored out the relevant tests.
Thus, I have not added new dependencies on the library (and I have even simplified one, as did others that forked this (data-default -> data-default-class)).

The quadratic test does take some time, but that is to be expected.

Hope this is fine with you,
Cheers,
Joao