First commit
Add tenant initialize method
Add tenant factory
Add credit_rating method to Tenant
Add Apartment class and initializer
Add tenants reader to Apartment class
Tenants attribute cannot be modified directly
Add tenant factory to apartment spec, and begin add_tenant method spec
Forbids adding tenants with bad credit rating to apartment
add_tenant method actually adds tenants to the list

* also fixes apartment factory with optional pre-populate parameter
Stops new tenants when apartment is full
Create basic remove_tenant method
Remove tenant by object reference
remove_tenant raises error if tenant-to-be-removed isn't found
Add evict method
Add apartment-wide credit score
Create credit_rating method for apartments

* set credit_rating and credit_score to nil if there are no tenants
Put credit_rating method in a module
minor minor changes