ENH: Generalize GA to work with MCF API
Right now the Google Analytics functionality is too specific to the Core Reporting API.  It should be generalized to handle MultiChannel Attribution as well.

For example, 

```
def _get_col_names(header_info):
    return [x['name'][3:] for x in header_info]
```

Would break because headers in are prepended with `mcf:` in the MultiChannel Attribution API.
