Handle client/server clock offsets globally

Fixes issue #32.

Introduce "intertwinkles.now", which calculates the result of "new
Date()" which attempts to correct for the difference between server time
and browser time (where the browser's clock is in synch to global time).

Transmit server "now" on each page load, and calculate skew as the
difference between that "now" and browser's "now".  This loses accuracy
due to transmit time.  We could do better by sending this timing data
over the socket connection, since that incurs much less latency; but for
most purposes it doesn't matter.

(Progressive Clock is already using its own socket-based clock
correction routine).