starts delete listing usecase and spec
Merge branch 'development' of https://github.com/felguerez/aircycle into delete_listing

Conflicts:
	lib/air_cycle.rb
adds specs and methods to in_memory to list all listings and delete listings
finished delete listing use case. success!
Merge pull request #49 from felguerez/delete_listing

Adds 'Delete Listing Use Case'

Great work, Brandon! ^_^