"greater than" comparison method of doesn't perform as expected on Micro objects
Using IPython:

``` python
In [1]: import pandas

In [2]: Micro = pandas.datetools.Micro

In [3]: 3 * Micro() > 2 * Micro()
Out[3]: False

In [4]: 2 * Micro() > 3 * Micro()
Out[4]: False
```

They can't both be `False`. Giving the arguments to `>` to `cmp` works as expected. Not sure where the issue lies since neither `Micro` nor any of its superclasses implements any of the comparison methods except `__eq__` and `__ne__`.
