Basic simpledb-based catalog coordinator

Here's what a `.zinc` looks like

```
[vars]
	aws_key = "key"
	aws_secret = "secret"

[storage.dev]
	url = "s3://zinc-demo"
	aws_key = "vars.aws_key"
	aws_secret = "vars.aws_secret"

[coordinator.dev]
	url = "sdb://us-east-1"
	aws_key = "vars.aws_key"
	aws_secret = "vars.aws_secret"

[bookmark.demo1]
	catalog_id = "com.mindsnacks.demo1"
	storage = "dev"
	coordinator = "dev"
```