Use findup to load NPM tasks to match node module resolution conventions

Hi! Ran into something that I think is a bug in Grunt, but I don't know the project well enough to say for sure.

When node loads local modules, it looks in ./node_modules, then ../node_modules, then ../../node_modules, etc. However, Grunt does not mimic this behavior when it attempts to load tasks within modules, which breaks that convention.

This pull request is an attempt to fix that. I wasn't sure of the proper way to do it, but this works for me. There might be a better way, though.