Timers can false start 1ms earlier than needed

( Related issue: https://github.com/joyent/node/issues/534 )

@bnoordhuis , you actually introduced another bug in https://github.com/joyent/node/commit/82e9da9

Lets say:
```
var _idleStart = 0;
var msecs = 1000;
var now = 999;
var diff = now - _idleStart;
console.log('diff', diff);
if (diff + 1 < msecs) {
  console.log('schedule for', msecs - diff, 'ms more');
} else {
  console.log('immediate exec');
}
```
Outputs
```
diff 999
immediate exec
```
which is definitely wrong.

There is a complete test for this bug. You can include into node/test/simple:

```javascript
var common = require('../common');
var assert = require('assert');
var i;

var N = 30;

var last_i = 0;
var last_ts = 0;
var start = Date.now();

var f = function(i) {
  if (i <= N) {
    // check order
    assert(i == last_i + 1, 'iteration order is broken on ' + i + 'th iteration, prev is ' + last_i);
    last_i = i;

    // check that this iteration has been fired at leat 1ms later than the previous
    var now = Date.now();
    console.log(i, now);
    assert(now >= last_ts + 1, 'iteration has been fired within the same ms as the previous on ' + i + 'th iteration');
    last_ts = now;

    // schedule next iteration
    setTimeout(f, 1, i + 1);
  }
}
f(1);
```

it fails on the current master:
```
1 1351075317097
2 1351075317103
3 1351075317106
4 1351075317106

timers.js:102
            if (!process.listeners('uncaughtException').length) throw e;
                                                                      ^
AssertionError: iteration has been fired within the same ms as the previous on 4th iteration
    at f (/home/wicked/Alawar/node/test/simple/test-next-tick-ordering3.js:41:5)
    at exports.setTimeout.timer._onTimeout (timers.js:195:16)
    at Timer.list.ontimeout (timers.js:100:19)
    at process.startup.processMakeCallback.process._makeCallback (node.js:248:20)
```

If I revert the https://github.com/joyent/node/commit/82e9da9 , the test passes.