replace grunt-bg-shell with grunt.util.spawn

with grunt-bg-shell the child stdout of the child process would be buffered
until the buffer is full and node would then kill the process
use stdio:inherit to allow 'rs' restarting
fix grunt server not running compile and watch

using async was a bad idea here. this way, the other tasks will now run after
the process is spawned instead of waiting for it to fail. by using
grunt.fail.fatal in the callback we ensure a proper message is displayed to the
user in case nodemon itself crashes
Merge pull request #39 from c089/replace-grunt-bg-shell-with-grunt-spawn

replace grunt-bg-shell with grunt.util.spawn