Improved usability (#23)

* Make gmjmcmc and mjmcmc much easier to use with fewer arguments.
* Split argument functions into separate functions for GMJMCMC and MJMCMC.
* Add option to set different priors on different transformations.
* Store coefficients for all visited models.
* Add the option to get renormalised model probabilities.
* Improved example likelihoods.
* More consistent return values from mjmcmc and gmjmcmc.
* More nonlinear functions.
* Proper parallel run functions.
* Proper predict, summary and plot functions.
* Add basic tests for summary, predict and plot functions.