Tweak to SstoreClient.
Re-engineered error-handling for SstoreClient class.

SstoreClient now throws Exceptions for socket errors and when it is unable
to parse the database response into JSON. This way, the calling program can
recognize the difference between a procedure merely returning 'failure' and
true error cases.
Altered bikeshare.py to reflect changes to SstoreClient.
Altered checkin function to take user_id as arg.
Modified Bike Checkin API function to call db procedure.
Implemented RideBike procedure in API.