New HTML info page for Windows

This PR includes a new HTML page, displaying the same content as `datadog-agant info`.
Technically, it uses the same tornado webserver and codebase as pup.

* For Unix, it is available if and only if pup is enabled. A link to this page is displayed on pup.
* For Windows (which is the main target of this PR because Windows doesn't have a `info` command), it is configured with `use_web_info_page`, which is enabled by default. (For now, Pup doesn't exist on Windows).

By default, the URL is `http://localhost:17125/status`.

Datas come from the same AgentCheck as the CLI info page, but it uses a new interface (to_dict) to be usable in a template.