unique_{any,common,diff} for vectors

recreating snafu'ed #1836

also fixes a bug in set intersection
Merge branch 'master' of https://github.com/JuliaLang/julia into setvector2
changing names back again
uniquediff->setdiff, implemented symdiff

Documentation for set operations expanded.

Note that symdiff on sets works, based on the code in array.jl. It'd be faster to use the single-pass algorithm on Sets, but someone else will have to write that!
Merge pull request #2048 from HarlanH/setvector2

set operators for vectors