Lots of big changes

Updated docs
Shift from None to 9999-12-31 to represent Current entries (Simplifies
    queries, recomended by Snodgrass)
Manager/Queryset methods to handle both time scales finalized:
    (during, current, active, active_durring)
save_during method added to save for a valid period, and resolve splits
Tests written for save_during
txn and valid dates made private/readonly. Users are encouraged to use
    only the provided methods to modify valid dates, and not modify
    tnx dates.
Default Ordering set to _valid_start_date
Lots of integrity checking on both txn and valid dates
transaction wrappers added to db modifying methods
mock temporal foreignkey added to test app
tests refactored with easy to use constants for time values
Additional active row added for Acme with name == 'Acme Corp'
new test object 'Sue' is born and dies.

[#63303980, #63421322, #64519630]