Look at allowing parallel downloads

[jrburke]

In #112, @guybedford suggested doing some parallel downloading for added speed. The relavent code is in add.js around line 89. Perhaps put the promises in an array, then use q.all() as the return value from that section.

Ideally this would use a connection pool too, we do not want to flood github with a bunch of requests at once. So, the above section should add the promises to a connection queue, and the connection queue would work through them, only doing X number at a time, say 4 for now.

[rafi]

Much awaited.