Add support for sorting query by rank ascending.

[abelsonlive]

This is certainly an edge case, but I thought it would be useful to test the search relevance by playing with the rank sort order.

[kvesteri]

Before merging pep8 compatibility should be fixed (see travis). Also I think it would be better if we didn't add new keyword argument but rather make the sort argument accept any kind of SA expression and change its default value to None.

Then if user wants to apply ascending sort he would do:

search(query, 'something', sort=sa.asc)

[abelsonlive]

Hey, sorry for not noticing this. will change asap.