Actions support for bti

Actions support for bti

This commit introduces a new set of actions for bti. Now one can track
her friend's and public timeline, as well as updating her status. It
means one can see her friends latest tweets, or track what's up in
public timeline.

There's also a new option called "action", various values are "update",
"friends" and "public". The "update" and "friends" options require
authentication, but public does not.

This commit also introduces a new dependency, libxml2.