Refactoring code from Ken.cpp into Ken.h
in preparation for implemenation of backward language model
from Xiong et al (2011)
BackwardLM now can calculate its empty state
Added CalcScore to BackwardLanguageModel
Adding unit tests to Backward LM
Work on BackwardLM unit testing
Checking in changes.

This currently doesn't compile.

Need to debug why.
Initial unit test for BackwardLanguageModel Evaluate method
Added slightly more unit test coverage for backward LM.
Re-enabled commented out code.

Hopefully this commit will now compile on all versions of Boost.