Wait for complete current app shutdown before starting a new build run

[istyf]

When air is used with applications that take some time to shut down, it can become quite frequent to see problems with file access, port availability and even lockups that require ctrl-c or worse to get out of. There has been a few releases with fixes in this area, but the problems sadly persist and have a negative impact on development flow.

Air should make sure that the currently running application has been able to perform any required cleanup, and has been completely shutdown, before kicking off a new build.