Fixed assertion error message
This is a pretty straightforward fix, message was simply inverted.

thanks!
 did you actually see this error? how?
 @jtratner It came up as part of a traceback in some code I was debugging... Was a pretty weird error to try to make sense of: 

```
AssertionError: Truncate: 2012-10-02 14:51:00 must be after 2012-10-02 14:50:50
```

Err... Yeah? ;)
 @ojh can you post code that generated it? maybe need a test for it......
 The reason we're asking is because if you see an AssertionError, the logic
is probably screwed up somewhere upstream (i.e., some function isn't
checking for bad input or is passing arguments wrong). Generally, users
should never see assertion errors.
 @jreback @jtratner Sorry for the slow response, it was late when I posted this last night, and I've been busy all morning today...

The code I was dealing with is a bit hairy, but the following should illustrate the issue clearly enough:

``` python
import numpy, pandas

rng = pandas.date_range('1/1/2011', periods=72, freq='H')
ts = pandas.Series(numpy.random.randn(len(rng)), index=rng)

ts.truncate(before='10/31/2011', after='12/31/2011') # No problem here

ts.truncate(before='12/31/2011', after='10/31/2011') # Switched dates, to illustrate
```

That last line then throws the following error (which is pretty much what I would expect, except that the order of the dates is wrong in the message):

```
AssertionError: Truncate: 2011-12-31 00:00:00 must be after 2011-10-31 00:00:00
```

The author of the actual code which resulted in this had assumed that the dates he was reading in would always be in chronological order - turns out this wasn't the case in one particular CSV file that got imported... Obviously, this has now been fixed with an appropriate failsafe added. I fully agree that users should never see assertion errors, but the confusing error message made the issue harder to debug for me, so I figured it might save someone else the confusion in future.
 Thanks - that's helpful
