fix weighted suffstats in normal.jl
rename invertedgamma to inversegamma
Added Normal-inverse-{gamma/Wishart}.
posterior_sample returns a Distribution with params sampled from posterior.
Added posterior_sample specialization for univariate Normal.
Added MvNormalSuffStats to store sufficient statistics for MVN.
Posterior computations for Normal-inverse-Wishart.
Handle Gamma prior on precision in conjugates.jl.
Placeholders for normalgamma.jl, normalnonconj.jl, and normalwishart.jl.