Add support for int8/int16 types. Addresses issues #9 and #42.

tests/broadcast-2.ispc

@@ -0,0 +1,12 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    int16 a = aFOO[programIndex]; 
+    int16 b = broadcast(a, 2);
+    RET[programIndex] = b;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 3;
+}