Add relations for the Student t distributions

[rlouf]

1. If $Z \sim \operatorname{Norm}(0,1)$ and $\chi^2 \sim \operatorname{ChiSquared}(n)$ then $Y = \frac{Z}{\sqrt{\chi^2 /n}} \sim \operatorname{T}(n)$. Falls under Add ratio distributions relations #65.
2. If $X \sim \operatorname{Norm}(\mu, \sigma)$, $\tau = 1 / \sigma^2 \sim \operatorname{Gamma}(\alpha, \beta)$ then $X$ is a random variable with a Student's t distribution. This one falls under Add more scale-mixtures #49.