Added npm "grunt" script.
Added npm "grunt" script.

Since GruntJS 1.0 was updated to support being run without a global
install of the grunt-cli package, an npm script can be used to run
grunt directly. This is nice because it doesn't force developers to
globally install the npm module if they don't want to.

To use it run the following command:

npm run grunt

jasco/jquery-powertip,stevenbenner/jquery-powertip,stevenbenner/jquery-powertip,jasco/jquery-powertip