Json data source

Consul-template should be able to use json files as the basis for templates (`{{json "ami-id@local.json"}}`). Why?
- Testing templates without a consul installation
- Node-local data for use in scripts (ip addresses, zones, tags)
- Bootstrapping consul itself with node-local data

Still to be written: tests, docs for this functionality
@cultureulterior just move it out:

```go
read, _, err := dep.Fetch(nil, nil)
if err != nil {
  t.Fatal(err)
}

if read != data {
  t.Fatalf("expected %q to be %q", read, data)
}
```

Does that make sense?