Fst decoder on aalto

A simple fst-based decoder with a simple confidence measure. Tested on some grammar based fsts. Should be much faster at the same accuracy when compared to the token pass decoder with lm transitions set to 1 or 0. Uses c++11, so added that flag for decoder CMakelists. See fst_test.cc for example use, the test is not built by default but can be enabled in the CMakelists.