Chrononaut

- Improves efficiency of `.versions(after=X, before=Y)` method for models with both a `RecordChanges` and `Versioned` mixin

This release includes the following features (as well as some general code re-organization for cleanliness):
- A new `RecordChanges` mixin for inserting a `change_info` and `changed` column directly into application models (vs. distinct history tables)
- A simple `rationale` context manager for simplified rationales when working, e.g., in an application console:

python
with rationale('The user asked me to!'):
user.email = 'updatedexample.com'
session.commit()

This release includes:
- An `append_change_info` context manager (for legacy backwards compatibility, as well as the ability to mark changes for a single object rather than within a context/commit block)
- The ability to set custom change info recording via a function specified in the Flask config `CHRONONAUT_EXTRA_CHANGE_INFO_FUNC` variable – this removes the need to subclass Versioned for simply adding extra change information. Note the interface for this feature is likely unstable and may be broken as soon as v0.2.0.
- Some general code cleanup and renaming