Update NEWS
Customizable priors (fix #16)

 - vague priors for baseline and difference parameters can be adjusted using
   the om.scale parameter to mtc.model
 - the prior for the heterogeneity parameter can be specified using hy.prior
 - mtc.hy.prior() is used to specify the heterogeneity prior, which can be on
   the standard deviation, variance, or precision
 - mtc.hy.empirical.lor() takes informative priors from Turner et al. 2012
 - starting values are generated from the prior and for the correct parameter
 - sd.d is added as a monitorable variable regardless of what the prior is for