remove unused variable
add method for getting Place from checkin annotation

I briefly considered using this in DisplayLocation.fromCheckinAnnotation,
but decided that constructing a Place from Gson for all objects on
load could be expensive
persist places for offline use when persisting display locations
cleanup, also never let name be null.
add CustomPlace, store them when found on messages
add clarification in javadocs for getPlace()
fix CustomPlace to make it easily gson-able
add method for excluding custom places
add method for getting ohai location annotation