add "blink forever" example

This is useful for quickly demonstrating the idle kernel.

examples/master/repository/blink_forever.py

@@ -0,0 +1,13 @@
+from artiq import *
+
+
+class BlinkForever(EnvExperiment):
+    def build(self):
+        self.setattr_device("core")
+        self.setattr_device("led")
+
+    @kernel
+    def run(self):
+        while True:
+            self.led.pulse(100*ms)
+            delay(100*ms)