merging on dates ignores sort keyword
```
import pandas
import numpy as np

from datetime import datetime

d = {"var1" : np.random.randint(0, 10, size=10), "var2" : np.random.randint(0, 10, size=10), "var3" : [datetime(2012, 1, 12), datetime(2011, 2, 4), datetime(2010, 2, 3), datetime(2012, 1, 12), datetime(2011, 2, 4), datetime(2012, 4, 3), datetime(2012, 3, 4), datetime(2008, 5, 1), datetime(2010, 2, 3), datetime(2012, 2, 3)]}
df = pandas.DataFrame.from_dict(d)
df.ix[:7]
var3 = df.var3.unique()
var3.sort()
new = pandas.DataFrame.from_dict({"var3" : var3, "var8" : np.random.random(7)})
df.merge(new, on="var3", sort=False).ix[:7]
```
