Finalizes the configuration of CASinoCore

* Autoloads the installed CASinoCore implementations
* Ensures CASinoCore.setup is called at the appropriate time
Removes database.yml from installation generator

CASino should not care about the contents of this file. That is up to
the host application to define.
Adds migration generator for CASinoCore implementations
Formally installs CASino into the dummy test application
Use in-memory SQLite database for testing

* Removes the /db*.sqlite3 entry from the .gitignore because that
folder shouldn't even exist if running the dummy app during gem
development
Fixes an improper engine mount location

The `:as` option specifies the namespace for an Engine's named routes.
By default, this is the same as the Engine's `engine_name` property. In
the case of CASino, this would equal `casino_engine`, which is the
"down cased" string value of `CASino::Engine`

In order to keep this both simple and consistent, the `:as` attribute
should be in lowercase. Otherwise, the namespace would look like a
constant which would be inaccurate.