Merge pull request #184 from Superbest/triangular-distribution

Triangular distribution
Distributions: cosmetics
Update contributing.md to clarify it is ok not to use real names
Special Functions: inverse of Gamma lower regularized
Tests: typo in test class name ModifiedBesselTests
Distributions: Gamma.InvCDF
ExcelFunctions
Fixed comment in TriangularDistribution example

The <summary> tag of the TriangularDistribution class read "ContinuousUniform distribution example". It now reads "Triangular distribution example".