Remove config/database.yml from git repo

I'm planning on making this a .gitignore file and allowing people to
configure their own version of this file instead.
Add config/database.yml to .gitignore

We want to force people to specify their own database config, so I'm
making this a non-indexed file. A subsequent commit will add an example
configuration as config/database.yml.example.
Add sample database configuration file

This file is meant to make it easier for people to get started. I've
only included a `development` section for now, but we could add more if
we need/want to.
Add original config/database.yml for reference

Since I've now removed config/database.yml from the git index, there's a
risk that people loose their database configs without knowing how to
recover. Therefore, I'm putting the original contents in a reference
file. We can probably remove this file when people have switched over.
Add database config file used by Travis

Now that there is no default config/database.yml file in the repository,
we need to make sure that Travis has one to use when running its test
suite.
Add section about config/database.yml to README

Now that we no longer keep this file in the source repository, there's
some manual setup needed.
Change to sqlite3 for the default dev database

It is way easier to get started with an SQLite database than a
PostgreSQL one. I just tried setting up a new project with SQLite and it
worked like a charm.
Merge pull request #88 from trotzig/simple-setup

Simple setup