Add parallel downloads

Uses PMAP gem to concurrently git clone repos.

Adds rakefile namespace :build and task :standalone_parallel.

Dynamically generates a standalone parallel infect bin.  Includes PMap
(parallel map) by querying Gem::Specification.  For the standalone_parallel task, it concatenates the Pmap lib with the infect standalone for a one-stop-script :).

Note: It can still use more work and tests, but it's currently working!  We can refine it from here.