Adds include for unistd for close and sleep calls

Older versions of g++ and XCode used libstdc++ and have recently
started using libc++. A consequence is that the functions in unistd.h
aren't defined implicitly, and they must be defined explicitly by
including unistd.h
And for sleep too
Merge pull request #6 from lukecampbell/master

Adds unistd.h include