Switch database to postgresql

To prepare for eventual launch on Heroku, configure the application
to use postgresql databases for development, testing, and production.
Add Article model

Add an Article model to the schema, with relationships to User and
Category models that don't yet exist.
Add ArticleCategory model

Add ArticleCategory linking table to the schema, so that each Article
is associated with one or more Categories, and each Category associates
one or more Articles.
Add Category model

Add the Category model to the schema, completing the association with
the Article model.
Fix relationship between Article and Category

Both the Article and Category models had missing has_many relationships
to the linking table, ArticleCategory.
Provide seeds for Articles and Categories in the dev database.

Use the Faker gem to generate data to seed the development database.