Export clock skew opinion as getinfo command

Torbutton could make use of a getinfo command to reduce the fingerprinting issues that a user has wrt their Date() object (see #2940 (moved)).

However, the threading support in Firefox is not good enough for us to easily make a full tor controller that listens for events. It would be much easier if the current skew (ideally down to milliseconds if possible) was available as a getinfo command, so that Torbutton could just query it and update the browser's time offset.

I am making this a 'major' enhancement because clock skew is an important issue to address to reduce fingerprinting.