Add rake task to setup DB layer for non-rails deployments

[wiggly]

Add a new rake task to set up db layers for non-rails setups and update README to provide at least one example of this.

[james2m]

@wiggly Thanks for this. I get the idea, I'd been thinking about dropping in an orm_adapters folder and requiring those prior to loading seeds.rake. There are currently some active_record environment dependencies in seeds.rake, I'd like to pull them out and keep seeds.rake clean.

If you were to structure this PR in that way, I could do the same for Rails do you think?