add Stan samples

samples/Stan/congress.stan

@@ -0,0 +1,14 @@
+data {
+  int<lower=0> N;
+  vector[N] incumbency_88;
+  vector[N] vote_86;
+  vector[N] vote_88;
+}
+parameters {
+  vector[3] beta;
+  real<lower=0> sigma;
+}
+model {
+    vote_88 ~ normal(beta[1] + beta[2] * vote_86
+                     + beta[3] * incumbency_88,sigma);
+}