Allow configuration of database connection to be used

Refactored the module to pull the connection resource name from the store configuration. The admin configuration is located within the *Reports* section and uses a custom source model to return the list of configured connections. This functionality allows one to create a separate database user with much more restricted access (e.g. read only permissions) and to use an entirely separate database for reports (e.g dedicated reporting slave host).

I added one TODO, the ability to override the database to query for each report. This would allow for reports to pull from multiple databases. This is probably a less common use case but I could see it being useful for some.