Ability to merge even if no common columns
Hi,
I would find it very helpful to be able to merge dataframes without common columns, (which is allowed in R), e.g.

```
x = DataFrame({'farm' : ['A','B','C']})
y = DataFrame({'crop':['wheat','millet']})
```

so that `x.merge(y, how='outer')` returns:

```
  farm    crop
0    A   wheat
1    A  millet
2    B   wheat
3    B  millet
4    C   wheat
5    C  millet
```

Currently pandas returns: `MergeError: No common columns to perform merge on`.

One work-around is to set the indices of `x` and `y` to zero, perform a `join` and the reset the index, as per this [StackOverflow post](http://stackoverflow.com/questions/13269890/cartesian-product-in-pandas). Another use case is [here](http://stackoverflow.com/questions/23331753/how-do-i-copy-rows-in-a-pandas-dataframe-and-add-an-id-column).

Alternatively, if there is a better solution, please let me know. Thanks!
