added json_safe_dict and to_json public instance methods to Review and M...

I added a to_json method (and a related helper) along with unit tests to both the Review and MultiReview classes for easy-exporting of the data returned by the Pitchfork client. For example:
```
# Review
import pitchfork
p = pitchfork.search('mogwai', 'come on')
#=> {Review instance}
p.to_json()
#=> {valid json string}
```

```
# MultiReview
import pitchfork
p = pitchfork.search('radiohead, 'ok computer')
#=> {MultiReview instance}
p.to_json()
#=> {valid json string}
```