Replace some rmath distributions

Since #138 got bogged down, I decided to separate out the pull requests so that they could be merged in piece-wise. This replaces the rmath code for the following distributions:
* Cauchy
* Geometric
* Logistic
* Log normal
* Normal
* Uniform
* Weibull

For these distributions, I've also taken the liberty of separating the code into sections
* Support: `insupport`, `minimum`, `maximum`, etc.
* Properties: `mean`, `var`, `entropy`, etc.
* Functions: `pdf`, `cdf`, `quantile`, `gradloglik`, `mgf`, etc.
* Sampling: `rand`, `rand!`
* Fitting: `fit` and sufficient statistics

I hope this will improve readibility and ease of maintenance: I'd appreciate any feedback/suggestions on this.