Re-enable backward LM code.

This commit updates that code to take into account Hieu's major refactoring.

Additionally, this commit re-instates changes from
commit 8459a86 (moses/LM/Jamfile, moses/LM/Ken.h, moses/LM/Ken.cpp),
commit d340218 (moses/LM/Ken.h),
commit 95e3a37 (moses/LM/Ken.h, moses/LM/Ken.cpp), and
commit aacc936 (moses/LM/Jamfile, moses/LM/Ken.cpp)

The above changes appear to have been improperly discarded
during the merge in commit 4894df6 and then again in commit 3ed17bb.