Add ability to configure hooks.

Adds custom callback hooks to be executed when triggered by a
POST to `/:suite/hooks/:group`.

This is helpful in our application to allow better integration testing
while still being able to reset the database at key intervals.

We are using the following:

```ruby
suite.hook :clean do
  DatabaseCleaner.clean
end
```

```javascript
window.cleanDatabase = function(suite){
  if (!suite) { suite = 'default' };

  jQuery.ajax('/teaspoon/' + suite + '/hooks/clean', {
    async: false,
    type: 'POST'
  });
}
```