fix: refactor code so most things work without sqlalchemy

also refactor tests to help ensure that remains true

Move tests out of the installed package set

this seems to be better practice. from what i can tell this is a
clean move with no breakage.. and built package files no longer seem
to include test modules

Make 'tests' a proper subpackage again; add some tests

Trying to make more of an effort here...

Overhaul config system, finally replacing edbob (yay!).

* No more `edbob.init()`, now we use `rattail.config.make_config()` instead.
* Adds new `ConfigExtension` mechanism to replace old 'init' config magic.
* Adds config extension for `rattail.db`, to load all Rattail db engines.
* All application entry points have been updated to use `make_config()`.

This also refactors `rattail.db` somewhat in general, to allow for safely
importing it without getting SA-related import errors.

General header / Unicode cleanup.

This hopefully updates all module headers such that the file encoding is
utf-8, and uses Unicode literals, etc.

File Monitor overhaul!

* New configuration syntax (old syntax still supported but deprecated).
* Class-based actions.
* Configure keyword arguments to action callables.
* Configure retry for actions.
* Add (some) tests, docs.