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...

General header / Unicode cleanup.

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

Database config/init overhaul.

This contains some not-very-atomic changes:

* Get rid of `get_session_class()` function and return to global `Session`
class approach.
* Primary database `Session` is now configured as part of command
initialization, by default.
* Make `config` object available to subcommands, and `Daemon` instances
(the beginning of the end for `edbob.config`!).
* Add `--stdout` and `--stderr` arguments to primary `Command`. These are
in turn made available to subcommands.
* Overhauled some subcommand logic per new patterns.
* Get rid of a few other random references to `edbob`.
* Added and improved several tests.
* Added ability to run tests using arbitrary database engine.

Removed global `Session` from `rattail.db`.

A Session class may now be had via `get_session_class()`.

Refactored model imports, etc.

This is in preparation for using database models only from `rattail` (i.e. no
`edbob`). Mostly the model and enum imports were affected.

Fixed bugs with `CustomerGroupAssignment`.

Now orphaned records should no longer be allowed.