Libcoap observer timeout fix (#137)

* Added -B flag to command
* Raise error instead of fixing the users mistake

pytradfri/api/libcoap_api.py

@@ -103,10 +103,13 @@ def _observe(self, api_command):
         """Observe an endpoint."""
         path = api_command.path
         duration = api_command.observe_duration
+        if duration <= 0:
+            raise ValueError("Observation duration has to be greater than 0.")
         url = api_command.url(self._host)
         err_callback = api_command.err_callback
 
-        command = self._base_command('get') + ['-s', str(duration), url]
+        command = (self._base_command('get')
+                   + ['-s', str(duration), '-B', str(duration), url])
 
         kwargs = {
             'stdout': subprocess.PIPE,