[WIP] Webhooks

Adds support for web callbacks in Iodized.  See: Issue #22 

TO DO:

- [x] Add persistence for webhooks
- [x] Add PUT, DELETE, GET support for webhooks
- [ ] Perform callback when an feature is added, modified or removed.

Mandatory gif:   
![gif](http://gifs.gifbin.com/092014/1411408222_pomeranian_vs_owner_eating_watermelon.gif)

/cc @madlep 



could refactor this to DRY `work?` for `:feature_table_created` as well.

One approach would be to pattern match on a tuple instead of a plain atom.

So 
```elixir
defp work?(:local, {:table, :webhook_table_created}, work_nodes) do
  ...
end
```

and
```elixir
defp work?(:local, {:table, :feature_table_created}, work_nodes) do
  ...
end
```

same for the `work!` definition