Add example for sorting by a dynamic Seq of columns

[cvogt]

review by @szeiger

[szeiger]

I'd rather apply sortBy calls for the individual keys in reverse order so you don't have to construct internal objects like Ordered manually and step outside of the type-safe lifted embedding API.

[cvogt]

@szeiger: I agree, updated the commit to do that

[cvogt]

@szeiger Please have a look. I'd like to answer some ML / SO postings with this.

[szeiger]

LGTM