Losing index and sort on merge
I'm almost positive I've filed an issue about this before, but I don't find it. This is a pretty common operation when working with panel data. You want to merge in some constant values into a DataFrame that has repeated keys but unique index.

```
df = pd.util.testing.makeDataFrame() # yes!
df["group"] = 0
df.ix[range(0, 20, 2), "group"] = 1.
df["time"] = np.random.randint(5, size=len(df))
df_x = pd.DataFrame(np.c_[np.r_[np.arange(5), np.arange(5)], 
                        np.repeat([0,1], 5)], 
                    columns=["time", "group"])
df_x["constant_info"] = np.random.randn(len(df_x))

df.merge(df_x, on=["time", "group"])
```

Where'd my index go? Where'd my sort go, so I could even recover the index?

```
[~/]
[65]: pd.version.version
[65]: '0.13.1-254-g150f323'
```
