Set npm package location to the root of the package

Fixes #24

This preserves the node behaviour, where relative requires
within nested main work as expected. E.g. a package with

"name": "mypkg",
"main": "./lib/index"

can use files from the package root in the main file like so

require("../foo/bar")

Other packages can read files from anywhere in the package
regardless of whether the main is nested, e.g.

require("mypkg/foo/bar")
Merge pull request #33 from KidkArolis/fix-npm-package-location

Set npm package location to the root of the package