WIP: Work on creating Retry

[AshtonKem]

First pass at creating a Retry mechanism for failsafe-rs. I took a stab at reusing the StateMachine functionality from the CircuitBreaker, and wasn't terribly impressed with the results. I'm curious about your take on this so far, since I'm pretty new to Rust in general.

What works:

• It does retries
• It fails after a configurable number of retries
• Tests

What doesn't work:

• Instrumentation
• Configurable delays
• Benchmarks

Affects #18