sails teardown should handle SIGUSR2 kill signal

runnig sails in development mode using nodemon can cause one extra grunt instance on every sails lift.

nodemon sends a SIGUSR2 kill signal to an app when it sees a file update.

this commit handles this kill signal and lowers sail gracefully.
listen for SIGUSR2 only once

this avoids a kill loop
Merge pull request #949 from edy/nodemon-support

sails teardown should handle SIGUSR2 kill signal