added a sequential impl of forward algorithm

a few notes:

- i made the styling tweaks @calvinmm and i agreed on over gchat. i also
  changed the indentation of `hmm.hpp` to 2 spaces to match
  `data_parser.*`. should've laid this out up front, my apologies.
- i removed the `state` struct from `hmm.hpp` since really the state
  also includes `transitions` and `emissions` and just threw everything
  in the `hmm` class. @calvinmm let me know if you don't like this idea.
- i tried to use basic loops instead of iterators since they'll be
  easier to convert to openmp pragma's
added another hmm test case
HMM implementation of forward algorithm

Test cases have also been added

closes #9