Add a way to resume started work

[kalon33]

After a lot of requests to OSRM, we got this reply from the server:

Traceback (most recent call last):
File "bus_router.py", line 599, in
getDirections()
File "bus_router.py", line 263, in getDirections
osrmDirectionsCall(stop, origin, dest, osrmpoints, fname)
File "bus_router.py", line 302, in osrmDirectionsCall
data = json.load(response)
File "/usr/lib/python2.7/json/init.py", line 290, in load
**kw)
File "/usr/lib/python2.7/json/init.py", line 338, in loads
return _default_decoder.decode(s)
File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded

There should be a way to resume geojson fetching instead of retrying from scratch each time there is a problem to complete.