grunt vs grunt-cli, notes about global installation

Changes done adressing the following comments
https://github.com/amber-smalltalk/amber-examples/pull/49/files#r18026252
https://github.com/amber-smalltalk/amber-examples/pull/49/files#r18026307
https://github.com/amber-smalltalk/amber-examples/issues/50#issuecomment-56827964