Add common functions and a Config class

I've removed the config function in the common.php and replace
that functionality with a class called Config.

With a Config class we have some advantages. The first one is that
now our Controller class have an reference of the Config object, so
its possible to access config items in any controller class and model
class. The second is that we can have multiple configuration files
and the method's in the Config class to load this configuration files.

I've added a function in the common.php file named load_class. This
function loads any class inside the core folder. That is nice because
all instances of classes are inside a static variable in the function
and we can get any instance of any class inside any other class.

We also have a function called is_loaded, that keeps tracks of all
loaded classes. This could be useful if we want the Controller class to
act like one big super object that can comunicate with all other classes
that are instantiate.

The bootstrap script is a little messy, I am still trying to clean up.
I am using now call_user_func_array in the bootstrap, so its possible
to have more than one argument inside controllers.

I've also deleted the welcome controller, model, and view, but that its
not important now, I was just testing some stuff.