Provide JS with reduced time precision

To help reduce information available to fingerprinting, we should randomize or truncate the values returned from Date(), event.timeStamp, and interval timers. I've never thought this was a useful thing to do before, because Tor latency is high enough and variable enough that most machines using NTP should be well concealed within the noise.

However, bug legacy/trac#1261 (closed) brings up a good point about javascript being able to measure the time intervals of various things (such as typing, but really it could be anything) to produce a fingerprint.

Unfortunately, we may need Firefox support for this, unless their javascript engine has changed to allow hooking of the Date() object again.