mbed PWM output to play siren

Revision:
0:ed48c1d4c02f
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed-os.lib	Fri Oct 21 07:35:16 2016 +0000
@@ -0,0 +1,1 @@
+https://github.com/ARMmbed/mbed-os/#a6f3fd1a60d5df59246d7caf3f108c4d34e1808e