Bayesian ANCOVA + small changes to ANOVA

Bayesian ANCOVA:
- Covariates are treated as nuisance factors, in a similar way to the
random effects in the Bayesian ANOVA

changes to ANOVA:
- only one random factor was supported for the right table output
- Bayes factors for Forward models are taken from the model output to
create consistent output
Merge pull request #38 from josineverhagen/development

Bayesian ANCOVA + small changes to ANOVA