Timedelta has unpexected properties when negative
Here's what's going on:

```
In [1]: import pandas as pd
In [2]: pd.__version__
Out[2]: '0.16.2+308.g529288e'
In [3]: dt = pd.Timedelta(seconds=-2)
In [4]: dt
Out[4]: Timedelta('-1 days +23:59:58')
In [5]: dt.seconds
Out[5]: 86398
```

I would expect `dt.seconds == -2`.

It seems like the fix may be something like:

```
In [6]: dt.days * 3600 * 24 + dt.seconds
Out[6]: -2
```

Is it that easy? `Timedelta` is based on UTC time, so days always have 24hrs, right?
