Refactoring of the chapter on probabilistic models

Changes to tcc.tex and nlp.tex were minor.

I added two papers regarding the Expectation-Maximization algorithm to
mendeley.bib as there's a footnote citing it in prob-models.tex.

There were three big changes in probabilistic models:

  - Added examples for Markov chains and HMMs, with figures.
  - Added the solution to the three HMM problems.
  - Expanded the section on HMM applications, with explanations on the
    two papers cited.