steam-api: do not directly access memory within json_values

Currently, the functions: steam_api_friends(), steam_api_poll(), and
steam_api_summaries() return GSLists that point directly to memory
allocated within json_values. While this is more efficient, it is rather
ugly. Moreover, with certain versions/architectures of GCC/glibc, issues
have come up during run-time.
steam-api: retrieve and process message timestamps
steam-http: implemented support for cookies
steam-api: switch to the Steam Web API

Valve has recently introduced a new web-based chat interface for the
Steam Community. The new web-based interface uses many of the same API
calls as the Mobile interface, however, all of the OAuth based calls can
no longer be used with a Web API token.

Since the Web API is designed to be used from the web-based interface,
someone hackery was required. The plugin now has to scrap the HTML of
the web-based interface to retrieve certain bits of data. Additionally,
the management of cookies is now required in order to correctly display
authentication credentials for certain requests.

Using the Steam Web API enables users to be shown as online via a web
client as opposed to a mobile client. Furthermore, the capability of
being able to retrieve unread messages is now available, and will be
implemented via another patch.

The downside of the Steam Web API is lack of information that is
returned for the summarization of a friend. Information such as the
game server and real name are not returned.
Retrieve unread messages upon connecting

As noted in the previous commit that implements the Steam Web API, the
ability to retrieve unread messages is now available. This patch makes
use of the new capability, and will retrieve unread messages from
friends upon the account successfully connecting.