add deleting files method

Usage:
```ruby
Parse.init :application_id => "<your_app_id>",
           :api_key        => "<your_api_key>",
           :master_key     => "<your_master_key>",
           :quiet          => true | false     

photo = Parse::File.new({
  :parse_filename => "parse_filename.jpg"
})
photo.delete
```

I didn't add testing code, because I have no idea about testing code.
If you access photo.url, you can see "Access Denied".
But I seem that we cannot test using instance variables of File class.

Would you give me some advice please?