Adding retrying transport package

 One retrying roundtrip implementation, some general purpose constructors and a test suite

* Use default HTTP transport or pass in custom implementation

* Setup acceptable status code ranges, retry on EOF, a delay time-base, max retries and global timeout

* Backoff-delay strategies: Constant, Linear, Exponential, Fibonacci or implement your custom delay function and pass it into the constructor

* Pass an event handler callback to do custom analysis of errors and intermediate (unsuccessful) responses

* Use convenience constructors for general purpose and most use-cases, adapt fields after construction if necessary