Add NPM postinstall script to check potential errors

I added a `doctor` helper to be runned on install script. I believe this is a good place to prompt users for potential errors in their environment.

With `yo@1.0.7`, we got a lot of issues opened with global generators not found because they have a badly setup `NODE_PATH`. Using this hooks to prompt user for potential error should leverage the number of issues opened, and help user found solution faster.

To test it on your machine, simply run `npm link` to launch the script. Then simply `export NODE_PATH=foo` between good and bad values and rerun `link` to check the output.