Parallel package installation

I just had to jump the version counter on this one, basically thanks to @elcuervo being an absolute boss johnny_deps now installs packages in parallel.

This should speed up significantly the installation of large dependency lists, he also did it with three lines of bash, which is just ridiculous.

This is not much of a benchmark, but on my machine and using my test Godeps file (consisting basically of random packages with random versions) I got these results:

johnny-deps 0.1.5 : 1m41s 
johnny-deps 0.2.2 : 45s

I love this man, give him your unending love and all the bacon he can eat <3.