Add support for vom Mises distribution

The initial implementation supports standard moments, pdf, cdf, and drawing
samples.

To support large concentrations, the scaled bessel function implementation
had to be used. This scaled version is currently not supported in Base,
such that its support is currently added only for the von Mises distribution.
Add von Mises distribution to documentation

Added description of constructors to univariate distribution
documentation. Also, renamed support to helper functions in vonmises.jl