New framework for AJAX services

New framework for AJAX services

This adds two new C4 modules, C4::Service and ::Output::JSONStream, and
makes important modifications to C4::Output. The first two are a basic
framework for JSON-based AJAX services and a simple JSON output wrapper,
respectively. C4::Output has been slightly refactored, with a new
function, output_with_http_headers, that supports different
content-types. output_html_with_http_headers still exists, and the three
pages affected by this change have been refactored to support it.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>