Add end point to get current stats on all clients

[thequbit]

This would include the total number of clients that have contacted the server within some time frame within a lat/lng box. This could then be used on the front-end to get an idea of how many people are in a users default geo-fence area.

[thequbit]

For the clients table we a number of things, but I think these are the ones we should return:

client_id = Column(Integer, primary_key=True)
cuid = Column(Text)

first_name = Column(Text, nullable=True)
last_name = Column(Text, nullable=True)
email = Column(Text, nullable=True)

verified = Column(Boolean)
verified_datetime = Column(DateTime, nullable=True)

creation_datetime = Column(DateTime)
last_check_in_datetime = Column(DateTime)

last_lat = Column(Float)
last_lng = Column(Float)

Will work on implementing this shortly. Nice to help track adoption rate.