Issue #140 - Add support for startTime and endTime to scanners

* Added StartTime and StopTime to ScannerOptions.
* Added extension method to convert a DateTime into an HBase timestamp.
* Added new options to ScannerOptionsConverter.

Also, there's no test support currently in place for `ScannerOptions` so I didn't add that in this change.
I'd go for a shorter naming convention here; something like `ToTimestamp` or `AsTimestamp` comes to mind.