Add parameter defaults for swaplevel

[brandon-rhodes]

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.

[jreback]

@brandon-rhodes yes this does seem ok