Added rake tasks to easily switch development databases

In order to make it easy for new contributors to get started quickly, while also balancing the need for testing in Postgres before deployment, this commit changed the following:
* Added two new database configuration files (database.postgres.yml and database.sqlite.yml), in order to maintain both Postgres and SQLite configuration options in the repository
* Added two rake tasks (db:select:postgres and db:select:sqlite) that copy the configurations file above into database.yml
* Added database.yml to the .gitignore file, so that contributors' database selections don't get committed and affect others