Project Euler: decent basic functionality tests & fun to write.

This seems like a pretty good way to get better test coverage of
miscellaneous functionality, especially of the mathematical kind.
That's how I discovered digits didn't work for BigInts yesterday:
the test introduced was actually problem 16.
Projet Euler 5: lcm(1:20) == 232792560

It's handy to be able to take the gcd/lcm of a collection of integers.
Also, code golf in your own language feels a bit cheap.
Project Euler 6-11.
gcd, lcm on collection: use reduce instead of splatting.
tests/euler.jl: 13, 14 & 20.
test/euler.jl: 24
Add summary of tools for enhancing performance
Merge branch 'master' of github.com:JuliaLang/julia
Performance tips: add pre-allocation
docs: fix formatting issues with 31c8388aa79366b1591464d5654ea0e3d2c2b518
Package libuv-julia-dev doesn't exists