Use Pkg.dir instead of explicit file path in examples

In `examples/probit_regression.jl`, for example, it is better to use

``` jl
vaso = readdlm(Pkg.dir("MCMC","examples","vaso.txt"), ' ')
```

than the current

``` jl
vaso = readdlm("vaso.txt", ' ')
```

so that the script can be run from other directories.

I can submit a pull request if desired.
