Factor resources, fix time

Now there is a generic resource that can generate and parse
application/json as well as text/plain. It can be re-used, only the
from_string and to_string routines have to be written and the resource
properly set up. A new resource format is specified, see
GENERIC_RESOURCE in, e.g., examples/osd/pwm-example. This is now used in
all my examples, namely pwm-example, arduino-sketch, wallclock-time.

There was an off by one error for the month in time formatting (in
gmtime and localtime). And the leap-year computation was broken. Both
fixed now, so we get a correct date. For localtime we are still 2 hours
off because daylight saving isn't implemented yet.

Also renamed gmtime to utc.

examples/osd/wallclock-time/er-example-server.c

@@ -286,7 +286,7 @@ PROCESS_THREAD(rest_server_example, ev, data)
 #endif
   rest_activate_resource(&resource_timestamp);
   rest_activate_resource(&resource_localtime);
-  rest_activate_resource(&resource_gmtime);
+  rest_activate_resource(&resource_utc);
 
   /* Define application-specific events here. */
   while(1) {