Use sqlite3 rather than postgresql

Not everyone has postgresql running, and getting it set up can sometimes
be a bit tricky.

The db/monster_demo is a sqlite3 database containing all the seed data.

This makes the setup process:

- clone the repo
- bundle install
- cp db/monster_demo db/monster_development
- rake db:test:prepare
Create pg_dump of the seed data
Update README with setup instructions
Add copy of postgresql config
Merge branch 'postgresql'
Make SQLite3 the default