Start using slim & composer

The current code has some charms but has some drawbacks as well. 

* While simple it also requires building some framework style bits.
* Duplicated code for bootstrapping and connecting to the database.
* Error handling and template adapters were built for xhgui but exist for all frameworks.
* Impossible to write tests for a large part of the code.

While not the final stage, this is a good first pass in converting the application to use Slim. I chose slim as it has the smallest footprint of the popular 'micro' frameworks I looked at. I'd like to convert the code further into controller objects allowing better organized code. Right now I feel things are a bit janky, and still hard to write automated tests for.