Clean up interface

Per https://github.com/gdb/configatron/blob/6c02f6f40b679a37848089c20a2c4423a52bd17b/History.txt, a summary of the changes is as follows:

```
This is largely a restructuring of existing functionality, making it
simpler and more consistent. The tests have also been fully
refactored.

Added the following:

* A Minitest integration at Configatron::Integrations::Minitest
* Key errors while locked return the name of the relevant configatron

Deprecated the following:

* Initializing a Configatron::Store with a custom hash
* Calling .nil?/.empty? on a Configatron::Store
* Calling locking, temp, or reset methods directly on a Configatron::Store (moved instead to Configatron::KernelStore)
* A DeepClone module is no longer defined at the top-level

Other backwards-incompatible changes:

* Renamespaced Configatron::Rails under Configatron::Integrations::Rails.
* Moved locking and temp methods to exist just on KernelStore.
* Configatron::Store.inspect no longer takes a name
* key? will now return true for sub-Configatron::Stores
```