coffee spawns own, uncontrollable child process

when using coffee as cmd the server is not restartable
workaround by using node with local coffe-script instead
(similar to [here](https://github.com/remy/nodemon/issues/195))

could be tested by comparing pid of child process on grunt/child side

without fix:
`
Starting background Express server
pid of serverchild on grunt side: 3048
pid of serverchild on child side: 8812
Express server listening on port 9000 in development mode
`
with fix:
`
Starting background Express server
You are using Coffee-script
coffee does not allow a restart of the server
found local coffee-script: ./node_modules/coffee-script/bin/coffee
using nodejs with local coffe-script instead
pid of serverchild on grunt side: 8416
pid of serverchild on child side: 8416
Express server listening on port 9000 in development mode
`