adding timestamp property and tests, prep for time-based checkpoint switch
delete duplicate get_docs implementation
move chunked and iter_docs to dimagi utils
adding datetimes to the schema_index view so they can be indexed

- move some export code into the checkpoint model
- update view references to query using new keys
- add reduce view
- support datetime based queries (though timestamp is never currently set)
switch to time based checkpointing
update schema checkpoints view to support timestamp-based sorting
clean javascript view code and remove dead stuff
add reduce to checkpoints view
add test for function that gets the last schema
Merge pull request #27 from dimagi/time-based-checkpointing

Time based checkpointing