Added Field#to_hash + tests + README example

This PR comes to you from the :us: federal government (specifically [18F](https://18f.gsa.gov) at the [General Services Administration](http://gsa.gov)).

I've found your gem quite useful. I added a feature that helps me quite a bit and wanted to contribute it back into the gem.

```ruby
fields = pdftk.get_fields 'some.pdf'
fields.first.to_hash
# which makes this easier:
fields.first.to_hash.to_json
```