create db folder with in_memory database file.
creates singleton for in_memory database
updates rspec configs to reset singleton db class before each test
adds database directory and in_memory file
-> in memory database can now create and get users
-> in memory database can now create and get listing_spec
in memory database can now create and retrieve reservations
>>Adds tests and database methods for adding and creating both reviews and messages.
Merge pull request #39 from felguerez/brady-in-mem-db

create/get messages and reviews