Add join to the pandas namespace
Currently `merge` is a function in the pandas namespace, and it is also available as a DataFrame instance method, with the calling DataFrame being implicitly considered the left object in the join.

However, the related DataFrame.join method, which uses merge internally for the index-on-index and index-on-column(s) joins, but joins on indexes by default rather than trying to join on common columns is NOT in the pandas namespace.

Adding join to the pandas name space will avoid having to write:

```
    if len(values) == 0:
        joined = None
    elif len(values) == 1:
        joined = values[0]
    else:   
        joined = values[0].join(values[1:])
```
