Update config.php

Add model_locations key
Add support for model_locations
Add additional directories

For #57 (and #58), tweak a bit.
Make the input option an array, so commandline can also add multiple
dirs. Add app/models to the config, prepend the base_path() to every
dir.
Check if the class is a model before instantiating
Tweak multiple models

Use array instead of comma seperated