If local grunt is not found and Gruntfile exists, npm install is executed

If Gruntfile is present, it is reasonable to assume that local grunt should be present and ```npm install``` can be executed if it is missing.  

Grunt-cli already fails if package.json is missing, so a check for it was not necessary.

Checking for Grunfile  and execution of ```npm install``` only occur at the point where the script used to fail, so there is no hit in performance to daily use.

This patch is the missing link for a single command for build since additions to Gruntfile can be made to manage packages after the initial installation.