Design choice of new timezone handing
Hi, just read the new article about timezone handing of pandas 0.8: http://wesmckinney.com/blog/?p=506

I see some issue of the design choice, and then I installed the pandas dev version to verify my guessing.

With the new timezone handling of Timestamp:

``` Python

In [18]: ts_orig = Timestamp('3/11/2012 04:00')

In [19]: ts_new = ts_orig.tz_convert("Asia/Shanghai")

In [20]: ts_orig
Out[20]: <Timestamp: 2012-03-11 04:00:00>

In [21]: ts_new
Out[21]: <Timestamp: 2012-03-11 04:00:00+0800 CST, tz=Asia/Shanghai>

In [22]: ts_orig.value
Out[22]: 1331438400000000000

In [23]: ts_new.value
Out[23]: 1331409600000000000

```

The problem was, a timestamp really should be a Unix time which is **"defined as the number of seconds that have elapsed since midnight Coordinated Universal Time (UTC), January 1, 1970"**. This means on every machine, unixtime or timestamp should be the same. **They do not changes.**

For different timezone, this is a representation problem, eg: if user changed machine to Asia/Shanghai, then we should display datetime to user with +8hours.

So, I think the correct and expected behavior maybe(iff timestamp really mean a timestamp):

``` Python
In [18]: ts_orig = Timestamp('3/11/2012 04:00')

In [19]: ts_new = ts_orig.tz_convert("Asia/Shanghai")

In [20]: ts_orig
Out[20]: <Timestamp: 2012-03-11 04:00:00>

In [21]: ts_new
Out[21]: <Timestamp: 2012-03-11 12:00:00+0800 CST, tz=Asia/Shanghai>

In [22]: ts_orig.value
Out[22]: 1331438400000000000

In [23]: ts_new.value
Out[23]: 1331438400000000000

```

This seems nature to me. But I don't understand the full design choice of pandas new timezone handing, so I open this issue for discussion.
- http://en.wikipedia.org/wiki/Unixtime
