Major changes:
```go
type ScanFunc func(r *Record) error
```
* Simplify the callback func signature by removing `ScanStatus`
* Leverage context for cancellation
* Add custom error `SkipCheckpoint` for special cases when we don't want to checkpoint
Minor changes:
* Use kinesis package constants for shard iterator types
* Move optional config to new file
See conversation on #75 for more details
