steam: implemented last online information

The last online time is sent via the user summary request, and has been
for quite sometime. The plugin should make use of this timestamp, and
show the user when a friend was last online.
steam-api: globally store all SteamFriends

This prevent the need to creating new objects all over the place for
SteamApi calls. Instead, the SteamFriend objects can just be updated
and referenced.