Fixed datetime-local to support DST.

Made changes to fix issues discussed in https://github.com/aldeed/meteor-autoform/issues/344. This removes the need to pass an offset if we're working with local time, but I suspect we should discuss the implications and whether these changes "fix" the issue for me but break the expected behaviour for everyone else. From my understand, `moment()` alone will handle all local time loading and saving without the need to call `zone()` or append a custom offset - both of which can cause issues for DST in the same timezone.
If I provide a date for DST in a location without an offset, the correct offset is set for my locality. However, if I impose the current offset (or a custom one) and I'm in Standard Time, then it is applied to the DST local time and the resulting time is off by an hour. I believe `moment()` alone will determine the correct offset for your locality and keep it consistent with DST.

```
moment('2014-01-01T01:00:00').format()
// "2014-01-01T01:00:00+11:00" - the correct local time of 1AM
moment('2014-01-01T01:00:00' + '+10:00').format()
// "2014-01-01T02:00:00+11:00" - the time has been altered since +11:00 is the DST offset in January for my timezone.
```