Update directive

Simplify directive syntax to accept a class or a hash including the class and options.

```ruby
sow [
  {
    :class  => Post,
    :source => open('https://spreadsheets.google.com/feeds/list/somerandomtoken/1/public/values?alt=json'),
    :parser => Sow::Data::Parser::GoogleSpreadsheetJson
  }
]
```

Add unit tests for anything refactored.