[WIP] Refactor to use dispatch_io for tasks, and wait for EOF

Hopefully resolves #151, #139, and #135.

/cc @joshaber @keithduncan
Ah, cool. I was just following (the dated) /var/folders/t6/tjsdgjqd6j7_vjgb66qvwlb80000gn/T/carthage-xcodebuild.kHa8tp.log.