csv exporter, config-file

added a csv-file exporter (writes into ~/.fitbit/$id/csv)

also added a config file reader (file: ~/.fitbit/config; see python/client_config.py)
defaults to no exports (no connection dumps, no csv), if file does not exists.
this was done so fitbit_client doesn't fill up the disk without the users consent (debatable choice, given todays disk sizes)

i don't usually program in python, so this is probably rather unidiomatic code