get service ID from datetime (closes #12)

add 3 functions to utils.py

getGTFSFileList looks in gtfsDir for files named like GTFS db files

buildServiceCalendar opens each of those files and concatenates their
calendar tables into a pandas dataframe (so we don't need to query
each of the GTFS dbs for every piece of location data)

getServiceForDate compares a datetime object to the start/end dates
and day-of-week service info in the full GTFS calendar dataframe and
returns an array of matching serviceIDs (I think there should only be
one matching ID when 0000 is excluded, but that's not required.)