improved show function for distributions
improved implementation of Categorical
extended tests on suffstats and fit_mle for univariate distributions, along with various fixes/enhancement on implementation
minor bug fixes for Categorical
improved implementation of Beta sampling
minor re-organization of binomial.jl
improved implementation of multinomial
minor improvements on Dirichlet
Some renaming of types for multivariate normal distributions

- The generic type that can take different covariance types is renamed to ``GenericMultivariateNormal``
- ``MultivariateNormal`` is now alias of ``GenericMultivariateNormal{PDMat}``, which refers to those with full covariance
- introduce ``DiagNormal`` as the alias of ``GenericMultivariateNormal{PDiagMat}``, which refers to those with diagonal covariance
- introduce ``IsoNormal`` as the alias of ``GenericMultivariateNormal{ScalMat}``, which refers to those with isotropic covariance
- introduce a convenient function ``gmvnormal`` to construct normal distributions of different types using simplified syntax
updated computation of sufficient statistics for multivariate normal distributions
initialize new API for conjugate modeling & inference
new conjugate API for beta-bernoulli/binomial
Merge branch 'dh/conj2' of https://github.com/JuliaStats/Distributions.jl into dh/conj2
new conjugates API passed for univariate distributions
partial conjugate prior on normal updated
updated normal-gamma & normal-inverse-gamma
new conjugate framework for normalwishart & normalinversewishart
minor bug fixes in normal.jl