ISSUE-2: Added domain classes Customer, User, Invoice
ISSUE-2: Added slick Table classes for Customers, Users and Invoices
ISSUE-2: Added skeleton DAO traits for Customers, Users and Invoices
ISSUE-2: Added some information to the README about the cake pattern and other futuer approaches
ISSUE-2: Added specs tests for Customer, User and Invoice DAOs and implemented the DAOs
Merge pull request #4 from lenzenc/ISSUE-2

ISSUE-2: Added domain classes Customer, User, Invoice