Add gth support to mc_compute_stationary

This pull request adds support for the GTH solution method that @oyamad wrote to ``mc_compute_stationary``.

We now have a new, optional, parameter for the ``mc_compute_stationary`` function, with fairly self explanatory settings:
* `mc_compute_stationary(mc::MarkovChain, method=:gth)` 
* `mc_compute_stationary(mc::MarkovChain, method=:lu)`
* `mc_compute_stationary(mc::MarkovChain, method=:eigen)`

If no method is specified, `mc_compute_stationary(mc::MarkovChain)` uses GTH by default, as this is the most numerically stable.


A minor point: Will you drop the `+` on line 100, which is redundant?

`x[k] += x[i] * A1[i, k]` -> `x[k] = x[i] * A1[i, k]`

Thanks.