Drop If syntax
Maybe there's already a way to do this, and there's certainly a safer and more robust/full-featured way to do this, but I just monkey-patched my DataFrames to have a dropif method because I got tired of typing.

```
def dropif(self, col, cond):
    return self.ix[~eval("self['%s']"%col +cond)]
```

In the spirit of Stata's drop and keep commands.
