Chrononaut

This release patches a bug sometimes occurring in the `diff` method.

This release reinstates the original `diff` method signature from versions `0.2.2` and earlier. It also adds supplementary `diff_timestamps` and `diff_versions` methods for all the different diffing needs.

This release changes the way the historic data is being stored. Instead of copying the versioned tables, a single table is introduced which stores the records from all versioned tables. The historic record versions are stored as JSON snapshots.
This release also introduces tracking insert operations alongside the updates.

- Fixes deferred model resolution (10)

- 6 Previously, all column indices were transferred to history tables. Now, certain columns can be excluded by setting them as, e.g., `__chrononaut_disable_indices__ = ['col1', 'col2']` on the model.
- 9 Previously, validators were sometimes transferred to history tables and sometimes not. Copying validators over to the history tables is not a desired behavior, since validators can change over the course of time. Now, validators are not copied by default unless the `__chrononaut_copy_validators__` flag is set on the model.

- 7 Addresses issue where primary key collisions can occur in chrononaut history tables. Allows users to set new `__chrononaut_primary_key_nonunique__` flag in model definitions.