Aliasing issue in Firefox 15

Aliasing `performance.now` to `now` causes an exception in Firefox:

```
NS_ERROR_XPC_BAD_OP_ON_WN_PROTO: Illegal operation on WrappedNative prototype object
```

So either `now` should call `performance.now`, or it should just use `Date.getTime()` and not bother about microseconds :)

(This is causing the white posts issue in the buddycloud webclient)
