Add "ecb" crypto mode support.

## What this is

We are adding "ecb" cipher mode support to the dukpt gem. This will allow support for a subset of TDES DUKPT devices.

## How to test

I've added a very rough preliminary test to "encryption_test.rb". A better test requires more time as the existing "cbc" tests are based on test data provided in the ANSI spec. The ANSI spec does not provide test data using "ecb" mode.