Moves grunt and friends into devDependencies block

Further to @rdepena's [comment](https://github.com/rdepena/node-dualshock-controller/pull/18#issuecomment-40746300) on #18, this pull request moves non-runtime dependencies to the devDependencies block.

It also uses the `^` version specifier instead of `>=` or `~` as that's what npm seems to favour at the moment.