Use a module to contain this credit rating business.

* Much googling was had.
* No man can count the tears I have cried.
* JK. anyway, it works now.
Add a BUNCH of methods to the building class.

* initialize
* add_apt
* remove_apt
* sq_ft
Added Building revenue method

* tiny commit because i feel guilty for having such a huge one last time
Add tenant listing method and grouping-by-credit-rating method to Building class
Add override option to evict tenants from apartment
Add method to Apartment class to evict all tenants