[OMNIBUS-232] Add parallelization to software fetching

I pulled over jkeiser's Parallelizer stuff from Chef (thanks lamont-granquist for pointing me to that.. it saved a bunch of boilerplate).

I set the default concurrency level for fetching to 10 threads but its configurable in the DSL as the `concurrency` key.

Please review and let me know if there's something I missed (or did wrong) :)


![selfie-1](http://i.imgur.com/eT4nOmE.gif)
(hacky hacky)

https://github.com/opscode/omnibus/issues/232