Add parameter defaults for swaplevel
I will be happy to write up a pull request, but first wanted to gauge the sanity of my suggestion:

I think that `swaplevel()` deserves default values for its parameters, just like its friends like `stack()` and `unstack()` and `sortlevel()` that also all take an initial `level` argument. I suggest:

```
def swaplevel(self, i=-2, j=-1, axis=0):
```

This provides the greatest symmetry with the other methods that operate on levels: they all, if no level is specified, operate on the innermost level as their default.

In the very common case where there are only two levels to the multi-index anyway, this would reduce this frequent operation to simply `.swaplevel()` or `.swaplevel(axis='columns')` without, I don't think, any more loss of readability than when `stack()` or `unstack()` fail to specify the level upon which they are operating.
