Random Sample from Discrete PDF

Generate random samples given a pdf. The x and p(x) are specified in the
arguments. See inverse transform sampling, gaussdis, gammadis.

Examples:
X = pdfrnd(0:.1:20, gammadis(3,2, 0:.1:20), 100000);
figure, hist(X, 100);
%X will be 100000 samples from gamma distribution with a/k = 3, b/Theta =
%2.

X = pdfrnd(-10:.1:10, gaussdis(2, 3, -10:.1:10), 100000);
figure, hist(X, 100);
%X will be 100000 samples from a gaussian distribution with mean 2, var 3.
%Should be roughly equivalent to X = sqrt(3)*randn(100000, 1) + 2;