shill: cellular: Add Cellular.ICCID property.

This CL exposes the SIM identifier in CellularCapabilityUniversal as a
DBus property 'Cellular.ICCID' on the device object.

BUG=chromium-os:33120
TEST=Tested the following:
1. Build and run unit tests.
2. Verify that shill includes Cellular.ICCID property on a
   cellular device when available.

Change-Id: I18a4363edb5b6fde180a3232be71c2e527c6c05a
Reviewed-on: https://gerrit.chromium.org/gerrit/28859
Reviewed-by: Gary Morain <gmorain@chromium.org>
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
diff --git a/doc/device-api.txt b/doc/device-api.txt
index 6f2fb3f..4843899 100644
--- a/doc/device-api.txt
+++ b/doc/device-api.txt
@@ -243,6 +243,12 @@
 			(Cellular only) The International Mobile Equipment
 			Identity of the modem.
 
+		string Cellular.ICCID [readonly]
+
+			(Cellular only) For GSM / LTE modems, the Integrated
+			Circuit Card Identifer of the SIM card installed in
+			the device.
+
 		string Cellular.IMSI [readonly]
 
 			(Cellular only) For GSM modems, the International