Improve how the result count headers get output

[johnbillion]

Work in progress.

In #49 support for the X-WP-Total and X-WP-TotalPages headers was added. There are a few improvements that can be made:

• The headers should be output by the Ajax handler, not the provider
• There should be no need for the provider to return the posts per page value as this is provided by the arguments
• The MediaList class can handle this instead of needing an additional MediaResponse class

[johnbillion]

I don't think there's anything left to do here other than testing and, as you pointed out, to get any other providers updated as necessary.