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
Addressing issue JuliaStats/Distributions.jl#223

The support of the distribution was not defined, as AngularDistribution or
CircularDistribution class does not exist, yet.
Merge remote-tracking branch 'upstream/master' into addvonmises
Merge remote-tracking branch 'upstream/master' into addvonmises
Removed besselix implementation, as now part of julia/base
Merge remote-tracking branch 'upstream/master' into addvonmises

Conflicts:
	src/Distributions.jl
Move von Mises RNG to Sampler framework

Introduced VonMisesSampler and VonMisesNormalApproxSampler. The latter cannot
be replaced by a Normal sampler, as VonMisesNormalApproxSampler additionally
ensures thatt he returned samples are within [0, 2pi].
Merge pull request #223 from jdrugo/addvonmises

Add support for univariate von Mises distribution