some small string function performance tweaks

document optional length argument to bytestring()
add performance tip on string interpolation. closes #2050
Update NEWS and docs for #5811
Add missing hyperlinks to NEWS
It was getting a little ridiculous having to change the URLs for the win-extras RPMs every couple days.

So now we're mirroring rpms that seem to work, and using jldownload to failover to our cloudfront mirror
base/primes.jl: generalize primes/mask to BitArray or Array{Bool}.

For smaller numbers of primes, the Sieve of Eratosthenes still seems
to be quite a bit faster. Perhaps we ought to use a polyalgorithm
here, only using the Sieve of Atkin for large N.
Merge pull request #5831 from JuliaLang/sf/mirror_winextras

It was getting a little ridiculous having to change the URLs for win-extras