Externalize database configuration and add Travis support.

This makes the database configurable via config/database.yml, which is simpler to manage and easier on the eyes. The .yml should never need to be modified, really, and if needed it's read with an ERB template, so you can write ENV values in it.

Added Travis CI to run tests to make sure that works correctly. Before merging, enable travis on [travis-ci.org](http://travis-ci.org) for this repo. There's a successful build in https://travis-ci.org/dblock/octomaps/builds/18555022, the badge will look like this: [![Build Status](https://travis-ci.org/dblock/octomaps.png?branch=database-and-travis)](https://travis-ci.org/dblock/octomaps)

It looks like the production deployment has a shared database.rb. That will need to change to a shared database.yml. I couldn't test it, but it looks like it will work.