grunt works fine when installed locally

A starter kit is more welcoming to new users when it minimizes the
work they have to do just to get started. Installing grunt
globally might amount to unnecessary trouble for some users. This
automatically installs the grunt command locally. It also calls
`grunt` when `npm test` is run.
Simplify README wrt grunt, bower

Since the user no longer has to deal with grunt and bower, replace
those command examples with a short concluding note.
Merge pull request #4 from jessaustin/local-grunt

Local grunt