NASDAQ ITCH 4.1: Add support for retail interest message

This add support for the Retail Interest Message a.k.a. Retail Price
Improvement Indicator (RPII) that was added on January 8, 2013 to the
NASDAQ TotalView-ITCH 4.1 specification.

Signed-off-by: Pekka Enberg <penberg@valotrading.com>
Merge pull request #157 from penberg/features/nasdaq-itch41-rpii

NASDAQ ITCH 4.1: Add support for retail interest message