support `paths` option

Ports @trevordixon's paths option to v2Dev.

I updated the grunt-browserify's devDependency for browserify to `>=3.21.1 <4.0.0`, which matches the string for watchify's browserify dependency. This way, people who install grunt-browserify are likely to run the same version of browserify no matter whether `watch: true` or `watch: false`. We should probably try to keep our version string roughly in sync with watchify's.

(If there was a way to avoid specifying browserify as a devDependency, and instead use watchify's install of browserify, that'd be even better -- I'm an npm/node newb so I don't know if there's a way to do this)

Closes #144