Fix markdown link syntax
Merge pull request #2989 from staticfloat/master

Fix markdown link syntax
base/primes.jl: faster primes(n) and isprime(n) functions.

Sieve of Atkin for generating primes:

	http://en.wikipedia.org/wiki/Sieve_of_Atkin

Code very loosely based on this:

	http://thomasinterestingblog.wordpress.com/2011/11/30/generating-primes-with-the-sieve-of-atkin-in-c/
	http://dl.dropboxusercontent.com/u/29023244/atkin.cpp

Miller-Rabin for primality testing:

	http://en.wikipedia.org/wiki/Miller–Rabin_primality_test
isprime(n): better witness choices for various n.

http://mathoverflow.net/questions/101922/smallest-collection-of-bases-for-prime-testing-of-64-bit-numbers
http://primes.utm.edu/prove/merged.html
http://miller-rabin.appspot.com
add eltype, last, and pop! for IntSet
a few other intset-related fixes and doc updates
add some IntSet tests
fix smallint -> BigInt conversions, especially BigInt(Int128)
fix #3032, bug in immutable constructor with too few args
factor: don't generate as many primes when doing trial division.
primes: implement and test prime stuff for BigInts [closes #3033]

It is not recommended to use this code to factor large integers. The
primality testing for BigInts calls GMP, so should be pretty good.
test/Makefile: allow mpfr to be tested in isolation.
factor: precompute and use a list of small primes as before.
pkg2 wip: refactor using fixed() instead of installed().

It stands to reason that we don't actually care about what is not
fixed when we're figuring out what should be installed.