Added generalized T distribution

I've added a generalization of the Student distribution when it is not standardized (i.e., mu != 0 and/or sigma != 1).  Details can be found in Wikipedia's article on the  [Student t-distribution](http://en.wikipedia.org/wiki/Student_t#Non-standardized_Student.27s_t-distribution) under the heading "Non-standardized Student's t-distribution."